My idea was really just to use FTO*tier instead of (0+FTO|tier) - this let’s you set the prior, but avoids sharing informatio across the tiers.
You are allowed to do this, but I agree this is usually not a good idea. I mean the model just to show the general pattern of converting the model, not to suggest a specific formula you should use.
This is most likely an issue with the model - your model has a lot of parameters, so it is quite possible not all of them are well informed by your data. I assume you get divergent transitions with lower adapt_delta - you may check out Divergent transitions - a primer for some strategies (that apply to all sorts of problems, not only divergent transitions). Generally, I would start with a subset of the data and a smaller model to let you iterate quickly and understand where the problems lie - such a big model as the one you have is almost impossible to debug.
Feel free to ask here if some of the suggestions are unclear.
Best of luck with your model!