Well as you said tissue dht is unknown unless you do biopsy or something like that. So if you dont want or cant do it 2.5 mg dutasteride or more is your best bet to be safe forever.
No. It is safe at 12.5mg or 25 mg daily and there are no studies long term but some trans have taken it for many years without issues for now. If you take 50 mg or more it start to rise side effects long term, especially liver issues.
E2 + dutasteride 2.5 mg or more + 12.5 mg bica daily + oral min 2.5 mg + tretinoin cream and topical minoxidil 5% in temples and hairline is what I think it can be one of the best hair loss stack we have rn IMO