I agree based on the few matches we played 3-5-2. We looked so damn heavy at the back. With the 4-3-3 we usally camp outside the opponents box if they let us, but with 3-5-2 It kinda moved the game to the midfield. Thats not necessarily a bad thing but from what i saw we couldnt really exploit the space that opened up behind their defence. We barely had any penetration at all.
I think that could be fixed with different forwards or practicing more but i dont think this is the time to experiment.
I would prefer 433 with Fati on the left or the 4312 with Griez instead of Fati. Its hard to say which of these two are the best.
It might not be the perfect system to play against dynamic, technical sides or the sides you are describing that would leave space behind.
It's not also a system the team is used to, as we have been playing 4-3-3 for ages.
BUT,
it's a good system to break down parked buses, especially the ones that sit in two low blocks in a 4-4-2, and be safe against their counters.
What a 3-5-2 or 3-4-3 gives you is a spare man at the back (out of the 3) to always outnumber the two nominal attackers that usually are the ones on which any counter would be built from those teams.
Then the wing-backs camp on both flanks (as becomes indeed the case in a 4-3-3 when going all out to attack but with massive gaps at the back), and if you have creative md penetrative passes can be made.
Remember Setien's first game against Granada. Granada was very disciplined and hard to break. Despite being the first game that such a system was applied, the team showed for the first time in a long while some good behavior in suffocating the opponent and letting them minimal chances to break. Execution in the final 3rd was lacking that day, but the whole image was promising.
Consider facing the same Granada during June/July the way we played at the end, and you would have the frustrating Osasuna, Atleti, or Bilbao home games all over again.
Setien had good ideas, he quickly realized he cannot rely on the personnel he has to employ them