Jotting down some pointers as a reminder to myself and for those who might find it helpful.
(I have not read the entire thread so I'm sorry if some of these points are redundant)

* I had old Octopus card that had not been used in >1000 days, which means it was deactivated and had to be reactivated at a service counter (went to Airport Express ticket counter at HKIA arrival hall) at a fee of 50HKD (or was it 15?) each.

* I had a couple other old Octopus cards which were also inactive that I had just turned in and gotten the remaining values back (possibly minus some nominal fees).

* The only way to top up / add value to the standard physical Octopus card is with cash, at a top-up machine at MTR stations, MTR / ferry terminal ticket counter, some convenience stores. One nuisance is that you can only add values using $50 or $100 bills.

* I had considered the mobile app options: standard Octopus vs Octopus for Tourist. The former was not a viable option for me, as it requires HK-issued credit card to my understanding. The latter is certainly an option for any visitor, but I read that there's a 6% markup on foreign currency exchange built into each transaction, which I thought was a ripoff.

* All things considered, I thought the best option for me was to just stick to the physical Octopus card despite the nuisance of having to top up from time to time using cash limited to $50 or $100... I felt this way, because I have an ATM card without out-of-network ATM fees and I plan to be back in HKG certainly within the next 1000 days so as not to have to worry about returning the card or worrying too much about how much balance I leave on my Octopus card at the end of my visit.

* As mentioned by many, vast majority of taxis are cash only (no credit card, no Octopus card). Certainly none of the Crown Comforts would take anything but cash. I think we only had 1 or 2 taxi rides where credit card was accepted... they were newish cars.

* However, you can use Uber to hail/book taxis and so you technically could use taxis in HKG without having cash. You have preset and metered pricing options for taxis via Uber, and the preset pricing is usually not bad... usually turns out a hair more than if you had not used Uber, but it's what I opted for each time I chose to use Uber for taxi. The Uber booking fee was only 7.50HKD each time. (In one instance, the preset price including booking fee, toll and everything was 310HKD from Wan Chai to HKIA. The metered price would've been 304, plus there would've been 6HKD per bag in the trunk... so in that instance, the Uber preset price turned out cheaper than if I had taken a standard non-Uber metered trip.)

* Airport Express one-way between Central (HK Station) and HKIA is $110. If staying in Admiralty or Wan Chai (or TST with intent of catching Airport Express from Kowloon Sta.), taxi to the station plus Airport Express is still the economical option for 1 traveler but, for 2+, just doing a taxi / Uber from the hotel to HKIA just makes more sense generally speaking imho.

* I may not have enough data points, but I felt a world of difference dealing with HK taxis in the world of Uber this time around versus 6-7 years ago when I just had to deal with the taxis without help. I used to have to deal with taxi drivers trying to get me to change my destinations, refuse to drive me on the basis of my destination being too close, and other shenanigan. Catching a taxi during afternoon changeover was particularly tough. With Uber now, everything is so much easier and within my control.

* Back to Octopus card / cash vs credit card... still not a world of difference in terms of places that do and don't take credit card compared to 5-6 years ago. But a little bit of progress. For example, Tsui Wah finally accepts credit card! (but sadly, their number of location has plummeted)