Hello @gabrielle, getting back to you on these questions:
To use the concat formula with linked fields, it is necessary to first use the join function as Frederik explained above. I’ve checked with the team, and currently, there are no plans to allow the use of the concat formula on linked fields, but I’ll add it to the list of discussions as a potential new feature idea.
Yes, we do plan to make lookup fields filterable. Here’s an issue to track the progress: Lookup Fields/Formulas should be filterable/colorable/aggregatable in the grid view (#676) · Issues · Baserow / baserow · GitLab.