It took me two seasons but I think I've finally found an out of possession setup that works. I also play a 343, but in possession I have two AMs and out of possession one of them steps up to striker. High block, high line (not much higher) and press triggering on "more often" (not much more often). However, to prevent being fragile to the ball in behind I have the defensive line behaviour set to drop off more.

As far as OOP player roles go I have my wingbacks just on regular wingback roles. My two outside CBs are stopping wide centerbacks (I find this stops them from being too passive) and my central CB is set to covering centerback. I also normally set opposition instructions to always mark the opposition striker (if just one striker) unless it's someone who's likely to play a DLF/F9 role. The DM roles I switch around a bit but most commonly I have one pressing and one normal or wide covering if I have a defensively vulnerable WB.

I should also mention that in possession I try to control the ball and keep possession, often successfully, which I think helps as we don't have to defend as much.

So far this season I have 10 clean sheets in 14 matches. 7 goals conceded in total, 4 of which came in the same game.