How hard would this be to change / use another store?

Depends on which functionality you want to keep. If 100% of it, no other triple store has it out of the box. Some might be possible to amend to have some of it.
If only purely standard SPARQL things (no extensions, no special services, etc.) then any standards-compliant triple store should work, provided it can handle our dates (they are wider range than many implementations - including Blazegraph by default) can handle. I don't think there is other non-standard thing that is not listed in extensions.

In T192786#4152277, @Smalyshev wrote:

How hard would this be to change / use another store?

Depends on which functionality you want to keep. If 100% of it, no other triple store has it out of the box. Some might be possible to amend to have some of it.
If only purely standard SPARQL things (no extensions, no special services, etc.) then any standards-compliant triple store should work, provided it can handle our dates (they are wider range than many implementations - including Blazegraph by default) can handle. I don't think there is other non-standard thing that is not listed in extensions.

Would any other standard triple store with just standard things work for most standard SPARQL queries then? (Ignoring services etc.?)
Is there a list documented of the things that we have modified on our version of blazegraph? such as dates and the services added?

Would any other standard triple store with just standard things work for most standard SPARQL queries then

Should work. I don't see any reason why they wouldn't.

Is there a list documented of the things that we have modified on our version of blazegraph? such as dates and the services added?

Yes, as linked above: https://www.mediawiki.org/wiki/Wikidata_Query_Service/User_Manual#Extensions

There are more things, but mostly performance tweaks not visible to the user and irrelevant for stores other than Blazegraph.