I recently went through a similar process as you - first EV and didn't know anything about Teslas. OK it's in Norway, but pretty much the same considerations. This place has been a gold mine. I in fact had a new Model 3 on order, but decided to cancel and buy used, so started doing my homework!
It's correct what people say that they just make changes on a continuous basis, so hard to know what you are getting, but with the Model 3 there has been one major facelift/refresh where a lot of changes came at pretty much the same time. That was officially for the 21 model (so cars actually produced November 20 onwards) and that got you the chrome-delete black exterior trims, power trunk, heated steering wheel, heated rear seats, double glazing, heat pump, new (much better, imho) interior, etc. That's why people are saying go for a 21 onwards.
Of course it's a fuzzy transition, and in the UK market in particular, I know from a previous thread there are some early 21 'mongrel' cars (mostly 70 reg but even 21 reg) that have some of the refresh stuff like heat pump, power trunk, but still rocking the chrome and old interior! So you need to check carefully the spec of any you are interested in if certain things are really important to you.
Go a bit further into 21 and UK market switches to made in China cars. Pretty much everyone agrees they are better built. Not so much that they are all better than all US cars, but US cars are much more variable - some good, some crap, while Chinese cars are consistently good. You can identify by the VIN which you can see in the front windscreen. US cars start 5YJ, while Chinese cars start LRW.
Go further still, well into 22, and you get the AMD Ryzen CPU upgrade and an improved 12V battery. These are nice-to-have if you can get them. I ended up with an early 22 (built December 21) and missed out, but it's not bothersome at all. Yeah sometimes it feels like the screen is a little sluggish, and I've seen the video comparison of how much faster Netflix loads on the Ryzen, etc, but meh - it would be nice but not a deal breaker! If you look at any you can see the car spec in the menu with a few presses and confirm whether it is AMD Ryzen or the old Intel Atom. You can also confirm things like the heat pump from there so useful to learn how to check that.
Finally most people say LR dual motor, and that's what I got, but don't rule out the RWD. Looking at your described useage, you will be fine with the RWD lower range, and how much four wheel drive and faster acceleration matters to you is up to you! Depending how your budget stretches, a RWD might get you into something newer, less miles and most importantly more remaining warranty, so definitely worth considering. Apart from the obvious battery, motors, speed, the only other difference in spec between RWD and LR is an inferior audio system and lack of front fog lights. So it's not like you go pov-spec if you choose the RWD, plus it has advantages in battery flexibility and efficiency as already mentioned.