Hey Anton, welcome! 
Honestly I think the search index text is making this way harder than it needs to be. You can skip it entirely and filter directly with your many-to-many relationship.
Here’s what I’d do:
1. New field on Users
Add a number field to Users, call it something like Selected Category ID. That’s where you store which category the user is currently looking at.
2. Let the user pick a category
You can use a simple Dropdown, or if you have 5 categories or less, check out my Pro Switch Tab component — works great for this.
3. On tap / selection
When the user picks a category, just do:
Update → Logged In User → Selected Category ID → the ID of that category
So “Music” = ID 3, you save 3. Simple.
Also add a Change Input Value → empty on each tap so the search text resets.
4. Filter the Image List
Two filters on the same list:
Current Event → Category ID → is equal to → Logged In User → Selected Category ID
Current Event → Event Name → contains → Search Input value
First one handles the category, second one lets users search by name within that category. They work together so the user sees only events that match both the selected category AND whatever they type.
That’s it, no more dealing with adding/removing text from a string. Way simpler to manage 
Let me know if you get stuck!