Advice on multi-lookup column list threshold workaround

Hello,

Situation:

We are tagging most our data in Sharepoint with project numbers via a multi-lookup column.

Now some lists start to reach the threshold limit, and you can't index multi-lookup columns.

Problem:

Losing the functionality to filter, aggregate or sum by the project number is not an option. Neither is using a single-lookup column nor keeping the lists small.

Possible Solution?:

Are there any known best practices, so that the user can still filter and our pnp-js and pnp-powershell code can make use of an indexed column? By archiving the lists and libraries will cap at around 10 000 - 50 000 items.

I was looking at making use of the term store, a workflow to automatically mirror the project numbers to the new column and a powershell script to update the term store.

But before doing this, I wanted to ask more experienced users, if this makes sense and if there might be other solutions.

Best wishes

Nico