Some ideas - not an issue #1
Description
Use JTDX instead of WSJTx as it is a better decoder and less cpu hungry (a lot less compared to WSJTx).
Or even better: use wsprd directly, then you do not need a gui ;-) and a lot better performance giving you 10-20 receivers/decoders on 1 RPi
See for instance the wsprdaemon project
Compliments to you : Very interesting project with very nice GNU radio setup, thank you!
As it all in all eats up quite some money including the 2 bandsplitters and 6 RTL-SDR sticks think of better hardware like KiwiSDR, a Hermes Lite V2 or even a Red Pitaya for the same money. And as I commented under your Youtube video, the Red Pitaya has been doing multi band FT8 and WSPR for many years already.
Love your choice of active antenna, just got mine a week ago and it already has its 2 loops ...