OK, finally some progress.
I realised that even if I had not touched the "fast idle" screw the PO or indeed the mechanic who attempted to settle this bike a few months ago (attempting, failing and blaming worn-out carb's bushings) could have done that. After my latest deep clean, the carbs breathe much better to the slides can be closed more.
To cut a long story short, I unscrewed the fast idle completely and that pretty much did it.
Now I can use the main idle screw to change idling speed and after warm-up it settles nicely around 1000-1200 rpm (on the tach).
So, this problem solved, I am getting closer, I hope to smoothing out everything else that is either wrong or inconsistent about it (slow return to idle, mostly). I won't bore you with it now, I have plenty of suggestions gathered from reading the forum and a list of things to do and to try, but i will probably post a detailed report at some point to serve as a reference for anybody who found themselves facing the same problems.
For those of you not familiar with PD46A carbs, here's what the "fast idle" screw looks like, viewed from below (bottom of pic is airbox side, top is engine side). It should barely touch a cam on the choke shaft, where the 2 halves meet with the spring mechanism. For now I have retrieved it almost completely, but once I have synced the carbs and settled them properly I will reset this as suggested in the manual.
Thank you for your help!