Thanks for the feedback - I think a mode like you're describing could be a lot of fun for players who are already loaded with credits.
What I'm suggesting is you set additional goals for yourself (beyond earning the needed credits) to each ship, that way unlocking them might feel more substantial. For example, reaching a given rank in trade before buying the next mining ship or killing x number of pirates for the next combat ship.
Oddly enough, I have actually done almost exactly this, and I'm still sticking to it to this day.
Basically, my rules were:
(1) I have to buy each ship in the game, in order of price, excluding the rank-locked ones;
(2) I have to use each ship for the Elite discipline it was 'designed' for (Combat, Trading, or Exploration);
(3) I cannot buy a new ship until I reach the next rank - at which point, I have to buy the next ship ASAP.
So, because the starting Sidewinder was classed as a Scout Ship in the original Elite, I used its Basic Discovery Scanner to explore (and also to do the 'exploration' training missions, like Salvaging a Black Box, or whatever it was back then). Once I hit Mostly Aimless, I allowed myself to buy an Eagle, which I of course used to do the 'combat' training missions, as well as a few assassinations. Once I reached Mostly Harmless, I bought myself a Hauler, and completed the 'trading' and 'mining' training missions, until I made it to Mostly Penniless, at which point I bought an Adder...
It's definitely given the game more structure, and I don't regret doing it - but realistically, it hasn't worked too well. Ranking up in combat is SO much harder than ranking up in anything else, I've spent most of my time in fighters... which is ok, but it does get a bit samey after a while. Likewise, ranking up in trading is so easy, I actually hit Mostly Penniless after completing the 'trading' training mission
practically on its own, and I had to break my own rules to complete the 'mining' mission before buying my next ship.
When I bought my DBX, at the rank of Surveyor, I used it to visit the Guardian Ruins, and I took Ram Tah's mission to scan the obelisks while I was there. I visited two or three sites, scanned a couple of dozen obelisks, then headed back... but when I handed the mission in, it gave me so much experience that I skipped Trailblazer completely, ranking straight up to Pathfinder! Naturally, this spoilt my carefully planned progression... so I just had to 'pretend' that I was only Trailblazer, and carry on as if nothing had happened. (Likewise, after reaching Colonia in my Asp Scout, I ranked straight up to Elite... sigh!)
I am now in a Vulture, in Colonia, trying to claw my way up to Dangerous... and I'm going to be here a while, I'm only at 32%! In fact, I've had to abandon my principles several times, and just game the system in an attempt to make some headway... and my refusal to grind doesn't help matters. Still, I'm determined to do it! I will
not buy an Asp Explorer until I meet my own criteria!
I don't know if I'll ever make it... but it's gotta be better than just mining LTDs, so at least there's that.