Okay, so I did a lot of research for this and I know there is no choosing what I can change even on low doses and that just slows down the changes but I also know that people go back for a shot every two weeks or so. So what would happen if I only did one very small dose of T? Would the changes be very minimal? Would estrogen start to refill my veins to even out the T after awhile?
I'm asking bc I'm not ready to fully transition if I ever will be. Right now I identify as nonbinary

Smaller doses of T aren't enough to shut down E production. Small doses would give you a few changes, but yeah, the T would be sharing space with your natural E. It varies person to person what that dose would be.

A single full transition dose (two week's worth if a shot) would give you some bottom growth, and the "T cold" which opens up your sinuses and causes you to have post nasal drip, it sounds like you have a cold. And that would pretty much be it. By three weeks everything but the (permanent!) growth would be a memory.

A one-month supply of (daily) gel would do about the same. Not much happens in the first month except for bottom growth, plus a bit of blood thickening (males have more dense blood than females) and maybe a tiny bit of muscle increase.

Yeah, one dose isn't nearly enough to do anything permanent and noticeable, unless your bottom growth kicks in really fast. Some AFAB non-binary people do stay on T temporarily to get the voice drop, body hair, and clit growth before going back to an estrogen-fueled system, but that takes a year or two for the vast majority of people.

A decent full beard generally takes several years, just like it does on AMAB teenagers. 

Stuff like weight redistribution, face changes, mental changes, etc. are all dependent on remaining on T, and slowly go back to where they were before after you quit. So if that's the only stuff you're after, a short term course would be kinda pointless and needlessly painful.

If you only ever did one small dose of T? It would be out of your system in a few weeks at most and you likely wouldn't get many changes in that period.

Instead of thinking about transitioning and all the medical details, I think you should spend some time thinking about your own goals. Ideally what would your transition be like? What would be the end goal for you?

Once you know that, go talk to a doctor who is familiar with HRT. They will be able to give you the best advice about how to achieve what you're looking for.