I work in the battery world but other opinions will exist! I'll dump thoughts in simple terms to try and make it readily understandable. Hope it helps.
LFP, lithium ferro-phosphate is a different cell chemistry, common in China and often used with high power low range applications (buses/trucks). LFP is different to li-ion but neither worse nor better. It charges/discharges very easily, has an exceptional cycle life (you can charge/discharge many times with very little degradation) but at a cost of being less energy dense, so you need more volume to fit the same capacity of battery. You can very roughly equate cycle life to total lifetime mileage of the car - more cycles is more miles before pack needs replacing.
This makes sense for SR+ model, it has space for a very large li-ion battery so using less energy dense LFP Tesla can still achieve the required energy capacity that the SR+ model spec requires. i.e. SR+ with LFP has a bigger battery volume than SR+ with Li-ion.
Interestingly because LFP can discharge so rapidly, Tesla could probably make this model faster without undue harm to the battery. I expect they don't do this purely to keep model performance differences as a marketing ploy, not that the SR+ is slow anyway.
The car probably has an internal buffer, i.e. unused capacity at both ends of the voltage range of the cell. So when you charge to 100% it is likely actually less than this in reality. Same on discharge, when it states 0% they'll be a little left - evidenced by Carwow driving for miles after hitting 0% in their tests. High buffer helps protect the battery in early days and then can be used as the battery does degrade to maintain the stated range. So as capacity diminishes, move towards filling the battery to the real 100% capacity. On the 0% end of the capacity, by leaving some spare energy in reserve, you avoid the car damaging the cells as it continues to discharge to varying degrees when not in use. I expect at some point the car will completely turn off to protect the battery which will need some additional measures to open the charge flap and reenable charging (guessing). LFP models will likely 'supercharge' more easily than the Li-ion cars, certainly at a cell level they absorb energy far more readily but the car software will control it. There is a relationship here though with capacity, a larger battery (LR/Perf) can charge faster than SR+ simply because it has a bigger bucket to fill, so even though the SR+LFP has faster charging capability, because it is a smaller bucket it may actually charge at a similar or slower rate to the bigger siblings. It should however charge faster than the SR+Li-ion.
80% is often cited as an optimal charge level. This is a compromise but a good one - the less energy you store in the battery the more relaxed it is and the less damage caused. Think of the cell like a balloon filled with water, you can fill it to absolutely full but it'll be very stretched and tight and will become weaker - best analogy I can think of! If you can get away with only 50% in the battery and comfortably do all your journeys, then do that and recharge to 50% each day, it will benefit the pack. 80% means you get decent range and helps not stress the battery. 100% is for the days you are doing a long journey and want decent buffer to reach the next charging stop - I've charged my M3LR to 100% twice times in 18 months and even then it wasn't really necessary.
Another aspect of 80% is this is about the point that the pack reaches full charge voltage and switches from a constant current to constant voltage phase. Details are easy to find on the net but in simple terms, charging slows down from about 80% capacity, you'll see this if you watch it at a supercharger. This is why if you can charge to 80% or less and have enough range to get to the next supercharger, that will probably be quicker than charging to 100% and putting less charge in at the next charger or destination when you don't care anyway. As chargers get busier you could hypothesise that Tesla may start nudging up the kWh price after 80% charged to encourage drivers to move off to free up pumps. Note at home on a regularly 7kW charger you won't see this slowing of charge as the rate is already very slow compared to what the battery is capable of accepting.
Charging to 100% for LFP makes some sense, as said they have impressive cycle life even with full charge and discharge, so it will degrade the cell but by an appreciably smaller margin than for the Li-ion pack. By way of example, li-ion based cells get somewhere around 500 cycles before they are judged end of life (which in battery world is actually only 80% of original capacity). LFP will often achieve 2000 - 5000 cycles. Hence why Tesla isn't bothered about you charging to 100% to maximise range as you'll still get more cycle life. You can do the sums but even with 500 cycles of ~200 miles range, that's a 100,000 mile battery that's only lost 20% of capacity -ish. With the SR+LFP model, you're probably going past 500,000 miles before that happens.
The remaining range of the car is a very challenging prediction that the car computer makes and will constantly update. For most lithium based rechargeable systems an occasional 100% charge helps calibrate the algorithm that determines capacity remaining (and thereby range). Otherwise the car is trying to track capacity without a good known starting point. That means it is constantly trying to monitor exact energy consumption at all times (including when not in use) and estimate what is left. Add in that it has to predict temperature as this has an effect of the battery and car efficiency, as well as the parasitic drains on the battery e.g. if it is very cold tomorrow, it won't go as far as it is currently predicting today. ICE cars are no different, they use more fuel for the same journey if it's colder, or if the driver simply chooses to travel more quickly. I suspect that people never gave much thought to the predicted range remaining on their old diesels as it made no difference if you just needed to refill a little earlier given how fuel is so readily available. Remember in the end the range is only a guess and it has no real reflection on the actual true capacity of the battery. I would argue that charging to full and draining to nearly empty to 'calibrate' the range is futile and only helps the human feel better, it doesn't make any difference to the battery itself.
So called phantom drain is a thing, the car uses energy when it's sat doing nothing. Especially if it *is* doing something like cabin pre-heating or Sentry Mode or downloading an update and so on. This is a bit different to your old ICE dinosaur which did tend to do truly nothing when turned off, but then it didn't get regular updates, watch for intruders, defrost the screen while you were eating your cornflakes and so on. Also remember that whilst you have 'paid for' the lost miles, by comparison you've paid a lot less for the actual miles you have journeyed compared to an ICE car so let it slide!
As many others have mentioned, once you're past range anxiety and switch to percentage remaining rather than miles, you'll quickly get used to charging when you need to and relying on superchargers for longer journeys. My own way of using my car is to charge to about 85% once a week timed to finish by the time I'm about to do a commute. This journey gets the car below 80% and I carry on through the weekly commutes. I can usually get a full week of work and back plus a bit of pottering at the weekend from a single weekly charge, particularly as it gets warmer. I tend to not let the car dip below 20% purely because of the reduction on the GO pedal but there isn't any reason not to go lower. You can charge more frequently without any detrimental affect, I just avoid it because it's not necessary and is one less thing to do. With what I now know, I could have leased an SR+ and it would have been absolutely perfect and not affected me very much at all - a few more charges on the long journeys but nothing that I couldn't have managed. Trouble is, now I've had the LR and got used to the performance of it, tough to go back to anything slower next time.
