Cool that you do both types of kicks regularly.
To help narrow down your choices, you can use your current fins as a stiffness reference, if want something similar or stiffer.
I like the stiffness of my current fins, but In all honesty have no basis of comparison and they're split fins. I don't think that I want fins that are softer (but that is just a guess). I like the slight negative aspect to their buoyancy if I stop my feet will ever so slowly settle. Neutral would be ok, but being positive would be unwelcomed. I think I want stainless steel spring straps. Yes, the last shape of the pockets is important as you illustrate. I know that I have no comfort issues with my current fins with their pocket design. My booties have bit of a low profile sole.
The length constraint is fairly real. My 26" duffle is my staple transport for many years. I would not want a larger bag to accomodate fins. I think less than 24" (< 610 mm) is a real packing constraint. So for example, Mares Quatro Excel Plus at 26" (660 mm) are too long as are Atomic Split Fins at 24-1/2" (620 mm). By length, some of the fins discussed by length that should fit are:
Apeks RK3 - 19-1/8" (485 mm)
Hollis F2 - 19-7/8" (504 mm)
Deep 6 Eddy Fin - 21-7/8" (554 mm)
Hollis F1 LT - 22-1/4" (565 mm)
OMS Slipstreams - 22-3/4" (578 mm)
Hollis F1 - 23-1/2" (595 mm)
I can't find how long the Dive Rite XTs are in size large....(anyone know?). So, it looks like there are a fair number of well liked fins that will fit.