I like Moondrop Spring Tips as well but often use different sizes, for me, L on the right, M on the left. Depending on the IEM, some IEMs I do L both sides, some M both sides. I don't get unbalanced sound from using different size tips, the most important thing is to get a seal for consistent sound and if it takes different size tips that should be fine.

Seal test here to check if you have a seal, try to make sure you pass that with an even ear to ear 50Hz tone and then see if they are still unbalanced.

Secondary consideration would be to get similar insertion depth as that affects the upper frequencies, but this is much less audible anyway.

It's also possible Spring Tips just aren't for you, maybe you need to try something else. They work for me for most IEMs but I have some that they just don't work well with, I also like Azla Crystal, Xelastec (these mould to your ears with body heat, may help), and SpinFit, primarily CP100/+.