I have gotten a little further into it, seems like there will be plenty of XP to open all the branches of the skill tree if you aren't just speeding directly through the story (maybe even if you are, I have a ton of XP I'm just hoarding until more upgrades become available to me).
I haven't switched gear based on enemies yet, but I'm not playing on the hardest difficulty. I suspect it would be helpful there, but I haven't needed it. For example, I have some enchantments that give defensive bonuses specific to the enemies I'm fighting in this one area, but I never bothered to put them on my gear, and I cleared the area out already.
The one thing I have switched on the fly is my artifact- just another piece of gear that gives you bonus skills/stats. I have been wearing one that gives extra defense and a bonus to the parry skill, but if I get dangerously low on health, I jump into the menu and swap it for one that lets me restore my health on the fly. But even that wouldn't be necessary if I was fighting more efficiently. Sometimes I just get impatient with groups of lower level mobs and start button mashing, which gets me into trouble quickly. With bigger boss-type fights, I focus in and it's fine, but the groups of enemies attacking at the same time will take you out quickly if you get sloppy.
So I guess I'd say that at least on normal/hard difficulty, mod tinkering is not required, but will help out. Also, it seems like the situational mods are useful for whole areas, so even if you switched them out, it wouldn't be every fight, more like when you change areas (I've spent a few hours in the first area, and a few hours in the second, to give you an idea).