Sounds like the issue started with installation of the pressure gauge.
Did you use teflon tape or some type of thread sealant that could have clogged a passage in the carb?
Has E10 fuel sat in the carb for an extended period?
Could the gauge be restrictive/defective?
Fuel pump possibly giving up? I just had a mechanical pump start to fail in the middle of resolving idle issues.
10*-12* initial timing should be fine and 32*-36* total timing.
The idle transfer slots in the venturi's should be set to expose .020" for a baseline.
Idle mixture screws are usually in the .75 to 1.50 turns out range.
The way it's seeking a high idle then falling off means it "could" be lean. Vacuum leak or idle mixture setting and/or butterfly transfer slot relationship.
Accelerator pump linkage should have a .010"-.015" play also. Too tight and the pump can weep fuel into the idle mixture.
All these settings should get you close......but if a circuit is experiencing blockage or there's a vacuum leak or fuel supply issue you'll have issues.
