I second what
@thelivingroom said. But it's also true that once you build the 500 series chassis and power supply, your stated budget only leaves you with room for a couple preamp kits at most. This would nevertheless be a good investment, in my opinion, as you could add more modules down the road when budget allows.
That said, I don't think there's anything wrong with just using your 18i20. Those are perfectly good preamps - no, not fancy high end studio eye candy, but clean and reliable - and I suspect that any sense of a recording made through them being "less than" (assuming suboptimal mic placement, etc is not the cause) might just be a result of the perception that this is a "cheap" entry-level unit. Personally, I had one in my home studio for a number of years and recorded lots of audio through it in that time, and while I admit that I did occasionally coax the preamps into making a bad sound, most of the time it worked every bit as well as a I needed if I was looking for clean, straightforward gain.
For your dynamic mics, especially if you've got something like an SM7b, an inline booster like a cloudlifter really should be in your setup, IMHO. Good news for your budget, DIYRE offers a 2-channel version in kit form (I think it's called the MB2) for the same price you'd pay for a cloudlifter.
And if, ultimately, you really can't see your way through with the 18i20 and feel that you can't work without an upgrade, my suggestion is to get a Clarett+ 8 pre on the used market (a reasonable step up from the Scarlett without taking you outside the familiar territory of Focusrite's software) and one or two of the DIYRE mic boosters, which will give you everything you want and need while keeping you within your $1k budget.
Anyway, sorry for the novel. The last thing I'll say is this: recording on a budget is easier now than it ever has been, and - this is just my two cents as I reflect on my own experiences - most of the time, it's not the gear getting in our way, it's ourselves.
