I don't understand this logic. Numbers wise, it is practically impossible for SF to be in government without one of FG or FF, neither of which will touch them with a 40 ft pole. Every poll points to FG FF and Indies, (rural Indie group in particular). It simply isn't there for SF in electoral maths. And honestly, I think they are happy to bank the 34% and head for the opposition benches.
Tbf, I think likes of McDonald and Doherty are dead keen to form a government but your core point is correct. They probably need to push on a few more percentage points. They seem to be stuck in 30-35 region for a while now.
If FF have a good election (looking unlikely) there will be pressure on them to break any potential impasse by having discussions with SF and they'd be in a strong position in terms of demanding cabinet posts and priorities in government despite being junior partner in terms of numbers.. SF would need FF more than FF would need SF.
There are plenty in FF (two Mickeys for instance) who'd be very reluctant to enter government with SF and would be quite happy to see any talks break down and retreat into opposition for a spell.