Build the index never finishes/gets stuck.

Have you tried disabling other plugins to see one of those causes this problem? Does disabling custom field indexing help?

What kind of content is there in these posts Relevanssi can’t index well? Just plain text or something more complicated? Plain products?

With Relevanssi Premium, I’d recommend giving the WP CLI indexing a go, but unfortunately, that’s not an option with the free version of Relevanssi.

Clearly something has changed for the worse but I don’t know what changed on your site, so I can’t tell why it’s not working. I’ve gone through all the common problems with the indexing and none seem to apply here.

You can try this:

add_filter( 'relevanssi_indexing_adjust', '__return_false' );
add_filter( 'relevanssi_indexing_limit', function() { return 10; } );

This will set Relevanssi to always index 10 posts at the same time. See if this helps.

Theme functions.php or in a code snippet would be fine.

Premium uses exactly the same indexing engine, so the results will, unfortunately, be the same.

There is something in your posts or in the process that slows Relevanssi indexing down.

You can find out the Relevanssi indexing MySQL query with this function:

add_filter( 'relevanssi_indexing_query', function( $query ) {
  error_log( $query );
  return $query;
} );

This will print out the MySQL indexing query to the server PHP error log. I’d index posts until it gets stuck again, then take the last query from the error log and run that on the MySQL server to see what it says – perhaps that will show what’s taking so long there.