I usually take real-world ranges from here:
https://ev-database.uk/
210 may be slightly low, but looking at what they quote for the other cars I’ve had, it’s not far off. The new WLTP range test is a joke, although admittedly not as optimistic as the old NEDC testing. As another guide I also use the USA EPA ranges, as I find these are much closer to what I personally achieve, but still somewhat optimistic. The EPA range for the SR+ is 239 miles.
If you’re happy to drive in ‘granny mode’ (no offence meant to any grannies out there) then you should be able to achieve the EPA rating, but I’m certainly not buying my M3 to drive like that!
And yes, you’re right, when you’re planning a long trip you do charge to 100%, but I wouldn’t want to be doing that week in and week out. I used to charge the Leaf to 100% all the time, as that still only gave me 80 miles max, but over the two years and 20k miles the battery suffered for it. I wasn’t concerned because I knew I was handing the car back after two years. With the Kona, I’ve probably only charged that to 100% a total of 10 times.
I fully appreciate what you’re saying about money. The M3 is a stretch for me too. I’m only passing on my own thoughts, and I’m sure you’ll be over the moon with whatever M3 you get.