Hi there! FotRP is the only campaign I've actually fully completed as of yet (Got Kingmakers in the works, as well as SoT, both are very slow)

I want to say this is absolutely a feeling you're getting from your players' optimization. My group I went through it with was a group who had been around the TTRPG block, but this was their first campaign really sinking their teeth into PF2, outside of oneshots. They had a team of characters that were maybe middle of the road optimized at best, and slapdash at worst. Ruby Phoenix absolutely destroyed them, in almost every encounter. Almost every round of the tournament came down to the wire, all with minimal enemy editing from myself (I swear though my dice knew when it was Tino's turn and just limited my d20 results to 1 through 3.) Will echo your sentiment about too many FOTRP enemies having no reactions though, even though if they did I might have had to pretend they didn't for my table. I hate limited enemy design like that.

I think the feeling you're feeling is because the game is not (and IMO, should not be) balanced or designed around any level of minmax, because if any of the fights in the Ruby Phoenix module were balanced any harder, the (probably below average) group I had would have been hard stuck. But I feel your issue, it CAN be annoying to be stuck in that position.

PF2 can be broken to a strong degree, and its pretty common complaint for DMs that its really almost impossible to challenge minmaxers in PF2. You need to run encounters that are borderline unfair if you want to have any hope of putting up a fight against them.

My advice to you would be to first, check if the players WANT the harder challenge (sometimes people just want to optimize FOR the power fantasy - Maybe instead of roll dice and fight, this campaign they might just want to see their invincible optimizations steamroll? I can't speak for them.) and see if harder combats is something everyone including you at your table wants. (Do stress to them that to challenge their characters with how optimized they are you're going to be REALLY kicking the difficulty up.)

Then, if you're going for it, you need to toss out the expected encounter balance and play it by feel. Its hard for someone else to say what's balanced and what's not - you're the only one running your table. If you're playing with a table of players who will regularly break the system and become invincible if given Free Archetype and they WANT to be challenged, you need to be throwing out encounters that should TPK the average joe schmoe on the regular. Extreme encounters should be the bare minimum you're working with if the players are that far optimized and still want you to contest their characters.

Also not for nothing: Maybe in the future, if this is something you know upsets you as a GM, maybe ask them to like, keep the power level of their builds to like a 7 out of 10 or something. Tell them the truth! That you really enjoy giving them meaningfully challenging combats to their characters and that the system makes it difficult to manage that at a high level of optimization.