Sheb – the ramban was most definitely a posek, and one of the gedolei rishonim on shas… he’s literally what we mean when we say “shas and poskim”, just like the ran, rashba, ritva, etc…..there were many communities that exclusively followed shitas horamban in halacha; i saw this mentioned in either shem hagedolim or knesses hagedolah; I forget which.
The ramchal didn’t write much on halacha, but he most definitely was a posek, as was any gadol beyisroel in those times….poskim take a lot of things into account when deciding halacha; the holy writings of the achronim who focused on machshava included.
The sefer ohel yaakov (rav yaakov sasportes) writes that shabsai tzvi introduced some positive things, such as saying birkas kohanim everyday in chutz laaretz. He writes that we specifically should avoid doing them, not to give credence to his heretical movement. If this was said about someone who believed in the torah, yet corrupted it and abused kabalah indefensibly, then kal vechomer to a group that rejects Torah and mocks rabbinic mesorah as being misogynistic.