Getting 2 specific stats in a specific range is definitely much more ressource intensive, than just getting a 60 in a specific stat.
Especially so, if you are looking for a narrow range of ±1 point or so.
I guess the difference between dumping heat and ammo vs heat and charge rate, is barely noticeable.
Both should be completely fine.
According to my own tests, 68 (possibly 67) thermal resist is enough.
Unless i missed something in regard to number of shots you can fire after x time of manual venting.
Since more heat resistance does not actually do anything as long as you have the minimum amount required to fire 8 shots before venting, heat resistance is definitely the stat where you can always to drop a few points, without geting any downside at all.
Charge speed does not make a massive difference, but it still has an impact on how fast you can put out your shots.
Iirc, dropping around 10 points on the ammo stat, will reduce your mag capacity by 2 shots (43 shots instead of 45 iirc) and remove a little bit of max ammo ofc. Not sure how much you need on the ammo stat, in order to get the max amount of ammo per ammo pickup though.
To me, losing 2 shots per mag is not a real issue. I usually get to reload before the mag is empty. And as long as i do not run out of ammo, the max reserve does not really matter that much either.
If you tend to run low on ammo, you might want to get all the ammo stat that you can get.