That's why I mentioned pairing cells from multiple used battery packs... This isn't magic, it's numbers... You pair like with like, and by removing the underperforming cells, you get rid of the dead cells bringing down the whole bank because they can't hold a charge. The fire risk is also greatly overstated... Batteries and management circuits have come a long way. You could swap in new cells and it would be fine... It would just be a waste, the new cells would degrade faster than their spec sheet implies.

And as far as matching like with like, you can build a lithium cell tester at home to profile each cell for a few dozen dollars... Literal hobbiest level stuff

And you most definitely can upgrade with newer, better batteries. There's three numbers - energy density, voltage, and discharge rate. You can upgrade the density to the moon, so long as the other two match it's a drop in replacement... And these three things are a tradeoff when you build the battery, so a better battery bank with the same output and voltage is just going to make you car run further, easy as that.

Yeah, TOS might stand in your way, but that's economic pressures, not an engineering issue. We could entirely solve that problem through force of law. On the topic, let me take this opportunity to promote right to repair - companies are going to feed us a lot of bullshit reasons why we need to throw it all away and replace everything fresh... They have every incentive to make us believe that. It's less work for them to say "that's not an approved use", they make far more money if they convince us they are the only ones that can fix it... it's an economic alignment problem, the engineering solution is well understood