^^^ This
tl;dr Just enjoy playing, whether that's RES hunting or hitting Combat Zones.
I did most of my initial ranking through RES hunting, but I started to get really fed up with the logging in & out to force respawning - i.e. finding the first pirate ship was an Eagle / Sidey / Cobra, exiting to main menu and reloading... rinse and repeat until you get a few Clippers/FDS/Pythons/Annies. I realise this tactic is widely used but it felt a bit exploity and certainly didn't help with the immersion feeling, so I stopped doing it. I was probably also starting to lose the love for the Vulture - it's just so agile and powerful versus NPCs, there wasn't any particular risk of dying, save for hitting an asteroid.
When I stopped forcing the respawns into big game, progression (I think I was dangerous by that point, but can't really remember) seemed so slooooow... in terms of bounties at least - which until PP was the only way of really tracking progress between ranks (albeit in a bit of a guesstimate way). So, I switched to CZs. By this time I had, in addition to my RES-hunting Vulture, a trade Python which was ready to be upgraded to a medium-level combat Python / multirole. I plumped for a mix of gimballed pulse and turrets and headed for a CZ. All I can say is I was rubbish in Open (I had to flee from a far better Vulture pilot) but in Solo it proved a steady earner, especially in conjunction with the couple of CZ community goals which were around at the time. Take it nice and easy, don't enter the middle of the fray, retreat to SC when you need to recharge shields etc. It was a different challenge - and more consistent - than RES hunting. I occasionally popped back into the Vulture to go RES hunting for a change, and also flipped to a bit of trading when I wanted to make more credits, more quickly.
The CZs helped me make it to Deadly and after even longer still, I had earned enough credits to buy a trade Anaconda (but trading in the Python). Perhaps foolishly, I took my new toy back into the CZ equipped with a very mediocre shield (all I could afford). With the rebuy cost being so high, I was very nervous so I interspersed the CZing with quite a bit of trading as I felt certain I'd lose my Annie at some point and wanted to have a few rebuys in the bank. Well, the Annie seemed to be an even better CZ tank, despite me not being able to outfit it very well. The big issue was I couldn't afford the PP/PD to equip very many / very powerful weapons, so I wasn't doing as much damage as quickly as I probably was in the Python, but shields held up well.
I think I must have been very close to combat Elite before moving into the Annie, as I eventually ticked over despite not doing too much CZing. I didn't find the Annie a good ship for RES hunting - largely because of the weapons limitations and the fact I couldn't afford decent thrusters, but I still had my combat Vulture which TBH is as good a ship as anyone needs for efficient RES hunting.
Not sure any of that helps, but my advice is to find stuff you enjoy, try varying it when it feels a bit flat and don't be too afraid of upgrading to a new ship when you can, if that ship fits (or will fit) with your preferred play style. My Annie, for example, is nowhere near combat ready, but I wanted to get it because I know it can be when I have another spare 180mn credits. Both RES hunting and Combat Zones will improve your combat rank but the single most important factor in "ranking up" is how much time you spend in combat. If you find something you enjoy, you'll spend more time playing, and rank up quicker each day/week IRL time.
