Yeah, I do differentiate between a Minor and Major Helm in terms of both top speed, and total size of ship they can move.
A Minor Helm can move anything between 1 and 50 tons, and has a top Speed Rating of 5. Spending zero or one spell level gives you SR 1, above that every two spell levels you expend increases the SR by 1 more. So, 8-9 spell slots produces SR 5
A Major Helm can move anything from 1 to 100 tons, and has a top Speed Rating of 10. Zero spell levels gives you SR 1, each spell level above that increases SR by 1.
There's also optional rules regarding acceleration in the guide I have written up so far...and a Major Helm gets to ignore those rules.
So my group just found our way into a spelljammer ship and are probably about to spend at least some time flying around our world's version of wildspace, and we were talking about how exactly to handle this. I like this idea a lot and will be suggesting it to the DM.
I might simplify things further though. You get an SR of 1 for "free" (ie cantrips). Then for every SR you want above that you have to spend the corresponding slots. So to travel at SR 2 you need to spend a 1st level spell slot, for SR 3, you need to spend a 1st and 2nd level, and so on and so forth, which means getting to SR 10 is possible, as long as you have spells at slots 1, 2, 3, 4, 5, 6, 7, 8 and 9 to spend. (and as you suggested in your original post if any slots in between are "missing" it drops the SR by that number).
I'd simply have a minor helm top out at 5 and restrict the kinds of ships they can helm and leave it at that, just to make it simpler to keep track of. If that means that at lower levels of spell slots a minor helm is just as good as a major one, then that's fine by me.
Anyway, I'm really looking forward to your guide! And I was very glad to find this discussion!