Well, you will have excess solar power during the day, so just keep it plugged in to the solar while solar is available. Then, just unplug the laptop in the evening until you get to 15-20%.
Trying to force the laptop to discharge while plugged in is colossally more trouble than it’s worth.
I would assume that they left the MX off of laptop GPUs, since they’re all MX cards, until recently. Regardless, the “card of the right approximate era” thing should work, unless there are specific patches for your card, which is unlikely.
It seems to me that the offending dialog would only be triggered if you did a full fresh install. During the previous iteration of the testing, they probably had a VM somewhere with it installed; since the underlying packages were already present, the dialog would never have popped up.
Ubuntu 16.04, dual booted on my laptop before I knew how much of a hassle that could be! Fortunately, never had any of the infamous issues.
A new iteration of open-source drivers for NVIDIA cards which aims to work better and be more feature-complete. Original announcement post here which explains a bit better.
…and for anyone like me who was unsure, yes it works equivalently for AMD. I think Intel as well, but I’m not sure about that.