I have been thinking of using FTS5 with SQLite to search emails. Not close of 43 GB ofc, so I would probably not have any major performance problems, but still is FTS5 any good? Or should I look into other solutions?
The nice thing about FTS5 is you can have full text search up and running on a dataset in a couple of minutes. So give it try and see if it is sufficient for your use case. If it is, great! If not, you've not wasted much time.
Have you seen 'notmuch'? I was honestly shocked how good/fast it is. It uses a C (++?) lib called 'xapian' to provide the actual index/search capability.
If you're talking about a normal local SQlite DB and your dataset is less than maybe 100GB of plaintext then SQLite FTS will work fine regarding performance.