I don’t have specific advice.
I would suggest researching trainers in your region whose riding style and student achievrments you like. Research then online to see if they have brought horses and students up the levels.
Many coaches get their gold medals on made horses, sometimes client horses, and haven’t trained above 3 or 4. Many coaches have a big grand Prix photo on their webpage but their students don’t go above 1 or 2.
When you find someone with the apparent qualifications, then go take some lessons with them and see if you like their teaching and personality.
Then you can broach the working student question.
The idea of starting regional is that you aren’t throwing up all your life and support system to end up in a strange town, strange barn, if something goes wrong.
If you find you have maxed out what you can learn regionally then you can start thinking about moving to a dressage concentrated area and a real BNT.
Rereading your post you say you ride “about” third level but don’t have a horse. I gather that you haven’t been showing to have that level confirmed or you’d have scores towards Bronze at least. So I take that “about” with a grain of salt.
If you want to be an assistant trainer you need to have some verified skills under your belt. You might or might not have these. If you want to be an assistant trainer primarily to get ride and lesson time that might be the wrong designation to aim for.
Perhaps what you need is a working student work to lease or part lease plus lessons until your skills are confirmed.
Most barns and trainers have some room in their life for working students but few except very large barns have any real “program.”
There is also a job classified site called Yard and Groom if you want to go national in your search.
I have to say it is rare for anyone to get truly accomplished in horses without buying their own eventually, even if it’s a project or green prospect.
