fb;
I don't quite agree with a couple of assertions in your posting...
"...enriching the mixture by one flat at a time to get to maximum revs"...all other things being equal and untouched, I would expect
leaning mixture to raise revs to a point...
"Several times the tickover dropped to 650rpm and stayed there till I blipped the throttle and the revs returned to around 800rpm." I would actually expect such an action.
"...mixture...related to the pistons not dropping correctly." ...height position of Dashpots is a result of amount of ingested air determined by Throttle. If Dashpots are free and clear and lubed, they will seek their appropriate height, and that will result in correct Metering Needle exposure and mixture. Base level of Jet sets mixture and I would not expect that to need more than a couple of flats of seasonable adjustment once set and checked with a roadtest.
"...what to try next." ...and you haven't given a reason or problem or symptoms why you are "tinkering with the mixture" in the first place...maybe you should start there!
...but first, may I suggest you have a look at the (Superb!) John Twist videos for a proper understanding of the function of your HS6 carbs: SU Part 1.
https://www.youtube.com/watch?v=7GRAcqDySog SU Part 2.
https://www.youtube.com/watch?v=60Bj_2cZQnc
Cheers