So I finished this off a few days ago. Difficulty actually levelled off after getting the 2nd element, aside from the very last puzzle in the game (which was insane) and the very last boss which was beyond insane... his phases had phases. Dozens of attacks. Took forever. There's a checkpoint halfway through and you'll need it for sure. Felt very satisfying though. I actually had to give up on it because I noticed it was about 5 hours later than I'd intended to stay awake for.
Loved the game so much I left a gushing review of it on Steam, which is something I've never done before. Often the rewards for finishing a quest or puzzle or finding a secret feel a little underwhelming, but there were a few nice rewards as well. Didn't even get close to 100% completion. There's a part near the end of the game where you can trade in the best hidden item in each area for a sort of key that unlocks a hardmode version of it- I found like 2 of 8, and one of those was basically a freebie. I wasn't scouring every last item intentionally, but if I saw an item or path I'd give it a solid try to reach it.
Game's combat is amazing as you get further in and unlock more abilities. You can specialize in all sorts of ways, and the different attacks really do have a lot of variety. It's not like 4 elemental versions of the same shit; as an example, the charged guarding abilties- the fire one counters an attack with a fuckoff huge laser that knocks an enemy back, does a ton of damage and burning status, and has a decent aoe along that line. The frost version freezes the enemy solid and does a chunk of damage, but has no AoE at all and only works vs melee. The wave version doesn't counter at all, but heals you for the damage you would have taken instead, and the lightning version also doesn't counter, but slows time for about 5 seconds. And those are just the level 1 versions. Theres 2 more levels to those, and alternate buff/aura versions of them at each level, and that's just for the guard type skills. Repeat for the melee, ranged, and dashing skills. Shit is bonkers. I get the feeling people will be figuring out new ways to speedrun this shit for years.
My main gripe would be that some of the timed puzzles are too strict in specific sections, and some of the ricochet ones as well. Though I might have been doing some of those wrong, because some definitely have alternate ways to solve them.
Also, the setting for the game being a game is brilliant, it lets them handwave a lot of stupid shit like NPC behaviour and arbitrary monsters and gamey world design while still getting a serious plot going on in the background.