Spotlight has broken down completely

MacOS Mojave Version 10.14.6 MBP Ret, 13-in, Mid 2014, 3GHZ Intel Core i7, 16GB 1600 MHz DDR3, Intel Irus 1536 MB - the spotlight index has completely broken down for some reason. Dragging the folder to Privacy tab and removing it again, does nothing. I have also tried to rebuild the Spotlight index from terminal using the zz folder technique. That does force the spotlight to reindex. After that, for a few days, the spotlight index seems to hold true and works - and then suddenly stops working again. I suspected an SSD issue so ran disk utility diagnostics which came back clean. I also reinstalled MacOS Mojave clean again. Still, the issue persists. The only thing I haven't done is a full erase and rebuild. I don't want to go nuclear like that just yet because I have huge amounts of data and personal setup on this machine that I'll need to rebuild. I'm afraid of doing a restore from Time Machine (TM) Backup after clean format and rebuild because that might bring the bad setup that has killed spotlight in the first place back. Cannot go too far back into TM because of the data I'll lose between that old working spotlight setup and now. In any case, it is a key part of MacOS and mustn't break down so completely like this!

For clarity, it brings back results from the internet and my internet history and so on but fails to search my files and folders locally. And no, these folders are not under privacy.