Hi,
sorry for not getting to you earlier. I don’t think you can easily define an array of variables in brms. There might be some options if you decide to completely hijack what brms does (as discussed e.g. at Brms & pk models? - #16 by martinmodrak). But I think you are running into limits of what brms can do for you and it might make more sense to just move to plain Stan.
Best of luck with your model!