First off, my main complaint with the original latin alphabet was that there were too many letters I didn't personally use a lot that also overlapped in function.
So I got rid of them.
A friend of mine showed me his alphabet, which was really useful and all-encompassing of the sounds you can make physically. But I my purposes here were to make easily writable letters, not necessarily a comprehensive archive of sounds. Also I wanted them to look aesthetic, so I couldn't have too many or they'd become complex.
The way he set up his vowels was really smart though, so I took inspiration from that.
He had far more, but I only cared about like, five vowels.
I laid them out by how they felt, like he did.