What part of a 3 turn cap would add much mental load? There could be a nice UI indicator to tell you how many tiles you have moved if you only move a few at a time. This also conveniently fixes strafe 2, no more time stop shenangians there as well.
Purple shrooms are far, far stronger for dex classes than mages during a fight. Mages get SP and MP back, Dex characters get cooldowns and SP. The cooldown reset is the most powerful thing purple shrooms does. It is definitely worth using SP to go pick them up mid-fight, and is something I use far more often on dex classes than mages. It is also % based, so dex classes have more range to go pick up shrooms, they will get more SP back.
You can shift click the character to rest to full, it should take less than a second?
The problem I see is any increase in SP regen will make it a lot stronger, even at the cost of reducing the max SP. SP is already one of the strongest stats, it doesn't need to get stronger.
To make it hard to get more than 5 or 6 SP would require a large reduction in the amount of max dex/SP giving items, which would affect how much classes have to invest in dex for sure. Replacing them with -SP regen items would also make SP stronger overall, something I am not a fan of.
I don't think SP stops being valuable, full dex is a viable build, but so is getting some str, or some int. In RF3 we had a big problem that dex was just the superior option, in RF4 the attributes are far more balanced. I worry that increasing the power of SP (making it regen faster) would upset this balance (maybe not for everyone, but the top players will be able to seriously abuse the game with faster SP regen).