the one thing is that if you never ever infiltrate or even support other FOBs, you're PF Rank won't really change it seems...
and the 'random 10 targets' for Infiltration or whatever seem sort of based on your PF Rank... if you're D rank, you're probably getting only 1-2 E rank players to choose from, and I think as time progresses (MGS V is less new) and more 'active' players are D or even C rank, they'll get E rank targets to choose from less and less
I think at release because basically everyone was new, so everyone was a target... but as active players level up, inactive E rank players should (...I think) be added to 'random target pools' less often... plus the hype of FOBs will probably wear down so it should increasingly make FOBs safer and safer as the game gets a older and active players outrank inactive-FOB players (...I think
just my general impression based on how my 'targets' have changed as I've begun to level up). You can still find very low level E rank targets but they seem to appear less and less now and I think it's because the matchmaking is trying to give active players more active C-D rank targets.
I turned off Reflex, Markers, Item HUD, etc all around 5 hours in, and I'm at about 140 hours now ;p I find it more fun.... I tried turning some back on and it felt a bit cheesy haha I mean that in jest, of course... I mean they were included for a reason but I do find the game a lot more tense, challenging, and fun without always knowing where enemies are, having a sort of 'get out of free card,' etc... I find the game not very tense around Mission 7 or so and always knowing where enemies were, and I found they were distracting, too -- I was basically just 'seeing' red triangles, instead of actually looking for the actual graphical movements of soldiers, heads or arms popping out, distant figures behind windows, etc.
nothing wrong with leaving them on of course (I mean, it's how the game was designed and intended, right) but I think I personally think it's more fun to turn everything in the HUD off, and I've basically went through most of the game (and S ranked a lot of Missions above 30) without any of the HUD, so they're definitely not required... (like, some games have missions where they're obviously totally designed and reliant on staring at a mini-map or some other HUD part; MGS V isn't like that, very doable and not unwieldy without a HUD).