The Abarth OT1000 & Abarth A112 engines have dish-topped style pistons that you can probably use as a "guide" to what works well. I'd start with machining your pistons flat to get rid of the higher-compression "step", then dishing them slightly from the center out to about 6mm or so just before reaching the outer edge, so there's the dish in the center & a flat "ring" around the perimeter.
For reference, here's a used set of original OT1000 dished-top pistons, where you can sort of see the flat outer "ring" I mentioned (see 2 pistons on the right):
View attachment 4579
And here's a birds-eye view as mounted in the block:
View attachment 4580
You may want to first machine your pistons to a flat top, then re-calculate (measure) your compression ratio; you may find this is enough to lower that 9:1 down to what you're after. If you still need to go lower, machine just 1 piston with a very (very) shallow dish, then remeasure. Repeat as necessary,
then machine the other 3 pistons to match. I have some OT & A112 pistons that I can (try to) measure the dish width/depth of, if you want? (Let me know)
Oh, & some various 850 compression ratio specs:
"Standard" Sedan 843cc engine 100G.000 = 8.0:1
"Super" Sedan 843cc engine 100G.002 = 8.8:1
Sedan USA 817cc engine 100G3.002 = 8.9:1
"Special" Sedan 843cc engine 100GB.000 = 9.3:1
Coupe/Spider 843cc engine 100GC.000/100GS.000 = 9.3:1
Sport USA 903cc engine 100GBC.040/100GBS.040 = 9.5:1
**Note that some of the lower c/r's also result from a slightly different combustion chamber shape (depth) between the Sedan cylinder head & the Coupe/Spider/Special cylinder head (the Sedan chamber being deeper). This is why Fiat had 2 different chamber-depth measuring gauges amongst the various OEM 850 "service tools".
The "Super" 843cc & USA 817cc Sedan engines got their compression bump mainly from the "step-top" type pistons, as they were otherwise a stock "Standard" Sedan engine (small valves, Sedan head & cam, single-barrel 30ICF carb). I've always figured that the .1:1 difference in their c/r's was likely due to the math of 843cc vs. 817cc measurements (rounding).