On your point 3 ( Tax residency: We don’t plan to live in Portugal even after getting the passport…). Neither the GV nor Portuguese citizenship oblige tax residency. Income tax applies to income of persons resident in Portuguese territory (over 183 days) and non-residents who earn income in Portugal. The official documentation is at: Personal income tax (IRS) in Portugal - ePortugal.gov.pt
On your points 4 and 5 - Legal firm selection and Fund selection - Jim Zhing (@jjmz) made an Excel file comparing a number of variables (Funds, legal firms, etc) which is available at Portugal Golden Visa Model - Google Spreadsheets. Please note that the file was relevant in July 2021 - so you will need to do your own research to verify the current situation, as some choices are/may be outdated/invalid. You may also want to read the postings at Real estate vs Fund option Cost Summary? as well as other more recent postings regarding fund choices.
Good luck.
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