It sounds to me like the disconnector or hammer has worn down, or maybe has broken, so that it’s no longer engaging and holding down the hammer. If that’s the case, then it won’t go into safe because the hammer has already dropped. Have you run a standard functions check? Obviously it won’t go into safe, but if you place the (empty) rifle on semi, pull back the charging handle to cycle the bolt, and then pull the trigger, does the hammer drop or does the trigger feel like it already fired? If it does click from the hammer falling, hold the trigger and cycle the bolt again. Release the trigger, and if there’s no “thunk,” then the hammer has already fallen in semi. Function check failed, and something is broken. Since you said you've fired thousands of rounds, it's possible one of the components reached its limit, or just failed prematurely.
It’s just me, but I personally don’t chamber a round unless my rifle is already on safe. Army training, and all that.