For dive courses, OW,AOW and then rescue/first aid, would be the minimum.
Then dive dive dive.
Specialties that enhance diving skills not underwater side activities. I would recommend Nitrox and deep for all divers with Dry suit, ice, wreck, cavern etc as interest takes you. Specialties are IMHO for activities that a new LDS will require you to show a card.
Equipment specialist if the workings of your gear and having the ability to do simple stuff yourself and save a dive. Science of diving if the theory really interests you.
I would stay away from photo, wildlife ID etc as you can get these skills elsewhere and at less expense. Just talk to other divers and perhaps pay for a dive and after dive beer to thank the mentor for the help. Take a particular specialty in this category only if it is the most economical way to get the skill set you want.
dive dive dive
Outside of diving, physical fitness, yoga breathing, seamanship, meteorology, oceanography, medical and first aid, etc will help.
DM is just about useless to make you a better diver. It needs you to be a good diver to start and teaches you to do those skills at a demonstration level, not a practical dive level. You would not take driver education teacher courses to become a better driver, so why take a dive teaching course to be a better diver? I looked long and hard at DM course and decided that my money would be better spent on just diving and doing other things, mentioned above, to make me a better diver.
One other thing to think about is that you may face increased liability in case of an incident, even if you are recreational diving as a customer of a dive operation. As a dive professional the level of ordinary care might be deemed higher for you than it would be for a ordinary rescue diver.
dive, dive dive
For what its worth, I am rescue diver with Ice, equipment, o2, nitrox, deep and dive science specialties. I am thinking about advanced nitrox and some more dive medicine courses. In the last year with a 30 day surface interval due to bad weather and a 120 day SI due to lockdown I will be over 200 dives. Targeting at least that in the next year.
Beyond that, dive, dive ,dive. Pay attention to your skills, practice buoyancy control, practice different fining and propulsion techniques, practice gas consumption management.
I would like to see Master Diver rating go away from being a customer loyalty card and be one where you had t demonstrate above average skills and knowledge at all dive aspects. Seriously limit the ability to issue this rating, perhaps to the course director level or employee of the dive agency to keep it from being another bragging rights card that anyone can buy.
Watch others underwater for good skills to emulate and bad ones to avoid, get a buddy with a go pro to video you and see if your skills look as good as you think you do.