Some useful info already in this thread, although this part is wrong:
Currently - fastest in terms of return: void opal minig. Get Python, slap mining laser, charge launcher, prospector limpet, refinery, collector limpet, load ~75% of your cargo space with limpets, find pristine metallic reserves ring, search for hot spot, drop there and spend few hours flying and trying to find VO asteroid with fissures. Crack open, gather all and repeat.
So small fact check:
- Void Opal mining can give good money: true.
- Python is a good choice: true. But about any ship of Cobra size or bigger can do it.
- Suggested equipment: not suitable. Take seismic charge launcher and abrasion blaster instead.
- Metalic ring: not suitable. Go for ice rings.
On the size of the ship, it's a bit of a tradeoff. I personally use a Cobra MK IV when going rock mining. I have to return to base more often, but considering that i often only have an hour or two for gaming anyway, that works out fine for me. While the smaller ship has an easier time going in between the pieces of the cracked rock. (Any ship can do it, but it's more enjoyable in a smaller ship. )
That all being said, i think you're not in that much need of making money. You have a T9 and a Krait MK II. So a good bulk cargo carrier and one of the best allrounder ships in the game. The Krait MK II is a reasonably good choice for about any activity in the game. It can fight (and even bring a SLF), it can explore, it makes an acceptable rock miner, etc. It's an extremely versatile ship.
Now the bad news: I think you're mostly fine in terms of credits. You might perhaps want to do a bit of void opal mining to a-rate your internals, but one or another hour might already do the job. The bigger climb is engineering. If you really want to get a ship, be it the Krait MK II or any other ship to be combat suitable, you by now should do plenty of engineering. At least shields and shield boosters should be engineered.
Mind you, a stock ship can fight. Whenever FD introduces a new ship, i first take a non-engineered ship to a HazRES for an hour or two, to see how it fares, to decide if it's worth the engineering effort. I never lost a ship yet when doing that, and i make sure to attack deadly and elite enemies during that test ride. But while a non-engineered ship can fight, it sure can be an uphill battle when fighting engineered NPCs.
As you don't have that much combat experience yet, you're well advised to first stick to only lower ranked enemies and/or invest quite some effort into engineering. I really wish engineering would not be as much power creep and not as essential, but infortunately it is. I am sorry for these bad news.
And a final small sidenote: the guardian FSD booster was mentioned above. While this is a good idea for somebody who plays a lot and travels far, i would not advise this for a casual player. You have to invest quite some time and effort to get that module unlocked. This means that for a casual player that module takes a long time to pay off. If it ever does. You might be much better off sticking with just a good fuel scoop and jumping a few times more, knowing that you saved a lot of time by not grinding at guardian sites, trying to collect the right data from the pylons.
[...]
It does require some grinding and there are guides out there on Google on how to do that. But I have found that the easiest way is just to do it passively. Don’t seek out the grind or you may burn out. Let the materials come to you naturally.
[...]
This method is slower than grinding but I find it a lot more fun than running circles in Dav’s Hope.
That's a good advise on material gathering in general. It's what i also currently do. I also collect almost everything when going bounty hunting. Once i a while i fly to a base with a material trader (actually i currently do my bounty hunting tours from a base with material trader) and trade those which i get plenty off for some which i am short of.
But you have to be aware that while it's more natural for the gameplay, it also is very slow. For somebody just trying to get one ship into shape, this can feel like an eternal thing and other alternatives might be more attractive.