Restore a newer filegroup backup
One of the considerations implementing multiple databases is different RTP/RPO requirements.  It seems this is the case in your situation because the RPO is different.  

Consider creating a separate databases for the ETL tables and historical data.  Backup the historical data database as needed to meet your RPO.  The ETL database can be backed up infrequently (e.g. only after schema changes), or not at all if you can simply recreate the database script.

Personally, I would split the database only if the size of ETL tables are disproportionally larger than the historical data.  The additional complexity might not be worth the effort, especially with backup compression.  If the ETL tables can be memory optimized, you have the option to specify SCHEMA_ONLY durability for those tables as long as you can run the ETL reset process after an instance restart.  That would avoid the additional backup overhead and allow you to keep all in a single database.