The maxest rank of all! And actually my motivation as well, just in a different way. Let me explain...
Totally understand what you mean, but for me to feel the 'achievements' in the OP as personally satisfying, I'd have to have done them 100% traditionally legit. It's the kind of person I am. I didn't, so don't derive satisfaction from them.
So, I stopped feeling a sense of achievement credits-wise at 700 million, because thereafter the min-maxing crept in (up to that point I was completely 1984, old school) and for combat rank the same would apply up to Deadly.
My motivation for maxing-out all six items in the OP was pure RL OOC results-orientated time management. Basically I don't have enough time for muh PvP so try to put the least possible time per year, for the maximum possible return, into those aspects of PvE that might assist the PvP.
I know that in the list in the OP some items (e.g. creds) might seem more obviously helpful to PvP than others (e.g. explo rank) but believe me all of them have an actual benefit. The only ones with no current benefit are the Admiral and King ranks, but they were 'future proofing'.
I maxed each one when specific game circumstances offered a perfect way to do it quickly and/or when there was a genuine side-benefit. Some are insufficiently helpful to be worth doing slowly and standalone, but were worth doing quickly with a side-benefit, if you see what I mean.
Well it's a personal goal, how you achieve them (and whether you can hold you head up high against your own personal standards) is up to you of course.
When I bought the game I knew I would want to get to Elite, because that's the point of the game, for me. Once I had the game I saw there were now three Elite targets, but exploration wasn't something I bought the game to do, and earning money had only ever been a means to an end in previous versions of Elite - you earned money by any means in order to afford the upgrades to make getting to Elite quicker, trading & exploring (to find new income sources).
In ED it started out that way, the only way to make decent money was by trading (hard work) or exploring (easier but slower) so just knuckled down & got on with rares trading & other things to be able to afford to kill NPCs more effectively.
But I also knew it was only a matter of time & persistence to get to Elite, if you stick with it eventually you will get there. It's not a test of skill particularly, it's a test of endurance, and I was up for that, so I challenged myself to get to Elite within a year, to re-earn my Acornsoft badge (my avatar).
As it turned out the game soon changed to make earning cash from combat much easier, and I abandoned trade & exploration, instead using the profits from bounty hunting to fund my upgrades. It made the game less deep, it took away the need to be a jack of all trades & allowed me to focus on optimising for combat, and I achieved my goal (Combat Elite) well before the other two. For the first time I did it without resorting to criminal activity, a personal rule I had not managed to stick to in previous versions.
So I set myself a stretch goal of getting the triple within a year of buying the game. I bought the game on the 13th January 2015, was Combat Elite by August and since then have lost pretty well all interest in combat. Between August & the end of the year I rarely had weapons fitted, I was either in a trade ship or a long-legged explorer, I didn't use any external tools or follow how-to guides, and while getting to Trade Elite (in 1.4) was hard work I wouldn't want to repeat, the final push to get Exploration (and therefore the triple) was one of the best moments I've experienced in a game. Tight up against a (self-imposed) deadline, I pushed far harder than I would otherwise have done, and achieved my goal with a week to spare.
I know what it took to achieve the goal I set for myself, it was a real challenge and I'm proud of myself for rising to the challenge rather than just giving up. Mostly I'm pleased that I found a target that wasn't too easy, or too hard.
Later I did a similar thing, taking my Corvette to Beagle Point, although I didn't set a time limit on that, I just knew it was possible in theory & pushed myself to achieve my goal, and derived personal satisfaction from an otherwise meaningless and arbitrary target.
I don't currently have another personal goal, I'm still on my way back to the bubble (very slowly) in my Corvette but when one arrives in my head I'll probably enjoy working out how to break it down into manageable chunks and generally being tenacious (my only real skill) to achieve that too
If you aren't proud of an achievement it's not an achievement, it's just a thing you did. Set yourself a lofty goal & a set of rules. Take a personal journey to push yourself and find out what kind of person you really are, and regardless of whether you actually manage it or not, be proud of yourself for not giving up.