Upstairs
2.5 Ton for the top floor (4 bedrooms), two returns (16x16 actual grill not opening) w/8inch and 10inch flex duct). The filter is 18x20. From what I've read I should have a 20x25 filter.
What I was thinking ideally was to run
3x8" returns from 3 bedrooms
1x10" return from 1 bedroom (master)
1x10" return (Hall / original)

How do I figure out the max size return plenum to buy (that would fit my unit)? Also when connecting to the plenum can I connect on any side or only on vertical surfaces (sides)

Alternatively if I merge the 3x8" and 1x10" into a junction box and run a 14" to the existing plenum how bad of an idea is that? Will still be undersized on the return

Downstairs
5 ton for the basement and main floor. It also has a 18x20 filter and ONE 22x22 grill (actual grill not opening). The return grill is near the stairs going to the basement with the main floor having an open floor plan. There are no returns in the basement (the plenum right before it hits the filter has a cutout and grill installed)

The plenum is half running behind drywall and connecting with a metal duct going to the upstair grill and would be somewhat of a bear to get to.

I was thinking of cutting a hole in the ductwork after the plenum and installing two returns to the basement area.
Regarding the plenum itself I can have someone try to remove and replace it with a larger one to accomodate a 20x25 filter but the problem is that would still be 50% undersized. From what I've read there should be two separate plenums coming out on either side w/a 20x25 filter on each. the other side of the HVAC is where the boiler sits. Also there isn't enough headroom to significantly raise the unit.
Googling I could find filters upto 24x36 on homedepot. Is it possible to connect a plenum of that size? Have the upstairs duct hook to it and the basements new ductwork as well

Here is a link that might be useful: