Glide Experts are finding ways to use relations to filter data using multiple choices. Kudos to them but it can be quite complex.
If you are prepared to compromise on App responsiveness by using a FILTER formula in your Google Sheet then this Google Add in for your sheet can be a big help.
I installed the add-in and it works beautifully - free to try for 30 days (no card) then 20USD per year. Perhaps the only negative is that you cannot use blank query fields. But if you dynamically filter using non-empty cells then this is great. It also handles (A or B) AND C logic.
Long term, we all hope Glide introduces Filters into relation look-ups!