I have their Plan B pattern supressor mounts on several of my rifles. They make a recessed version that really gets the suppressor close to the handguard. Here's a 11.5" handguard with a 12.5" barrel with the recessed version.
On the Geissele set, how tight does the upper and lower fit together? Is the upper receiver dry lube coated on the inside and have a thermal fit for the barrel extension?
Without breaking the bank, I'd look at the BKings Firearms M4 Mod-0 receiver set.
https://bkingsfirearms.com/shop/ar15-lower-receivers/bkf-m4-mod-0-stripped-receiver-set-2/
Here are 2 groups I shot with my WOA 18" SPR barrel while doing some load development. I didn't use anything special really. 1x fired PMC brass, Unis primer, 69gr RMR bullets, and 24.5 gr of Varget. Both groups are right at .75" at 100 yards.
I'm betting the spiral fluted barrels were originally made for the Stag Arms Pursuit rifles. According to the Aero rep I spoke with at the AZ CANCON, the Solus and Pursuit share the same action with only differences in external machining.