It sounds very much the state of your particular engine that's causing your issues, and is unindicative of the SF on general.

The SF is inarguably less powerful than the its supercedent FX/AX and alternative equivalent TT Pro, but it does punch out power sufficient for the larger sized props you mention. I've had running reliability problems with a next to new OOTB but old SF myself. The problem will prove to be either the bearings, rubber (gaskets) &/or carb contamination (gum or dirt) ....possibly a combination of all three. Though it's had little running, it needs an overhaul. So does yours by the sound of it.

The SF throws an APC 11x5 with authority, as it does a APC 10x7. Throttle response should be crisp & smooth in the usual OS fashion. I go up ½ inch in diameter and slightly broader heavier blade in a Bolly prop for the FX/AX and Pro.

Once overhauled, a 11½x4 Bolly if they make it in that pitch sizing (?) or 11x5 APC should work a treat with that engine in SIG's SE. A 10.5x5 Bolly or 10x5 APC is too small for that engine IMO.

Your real decision is whether to overhaul that engine or triage it to the bin and buy a Pro 46.