Plumbing advice

First, thank you all for the super useful info on this site. I wish I had discovered this site prior to having my pool built back in 2012, which leads me to these questions. I attaced a diagram of my pool layout. I "think" but not 100% certain is that the bottom drain was run with a 2" flex pvc pipe. All other plumbing was run with 1.5" flex pvc with the exception of the middle left return (it was a plumbed for a polaris pressure vac, but I repurposed it as an extra return. The pad is aprox 20 ft diagonally from the pool. Even though I think the bottom drain was run as 2", coming out of the ground the plumbing is all 1.5" PVC. To add to the cheapness of the PB they put in a 90sq ft cartridge filter.

I recently put in a VSP (SP3400VSP), and to get relatively decent/equal skimming action, I have it running @ about 2450 RPM with bottom drain 20% open, far left skimmer fully open and near left skimmer about 50% open.

Questions:
1. I feel like I should change the above ground plumbing to at least 2". Is it worth doing that? Should I go to 2.5"?
2. I feel relatively comfortable taking care of my parent's DE filter, so am thinking of moving to a 48sq ft DE filter, thinking 60 is overkill given my plumbing situation. Do you agree?
3. Should I try to dig down to where the 1.5 riser connects to the 2" bottom drain and go up with 2"/2.5" from there, or given that I am restricting the flow anyways to get skimming action I should not bother and just connect with the bigger pipe to the 1.5"
4. I may want to go back tho the presure vac in the futuere, would my 2x 1.5" returns be sufficient, or should I forget about the pressure vac and use the extra return?

Any other advice you would give?

thanks for all your help!