Is she just peeing indoors or is she pooping as well?
If she is just peeing, see the vet. She may have a urinary tract infection.
If she has no physical issues, then you are absolutely on the right to turn to remedial potty training.
If this dog has free run of the house, that stops now, since she is not trustworthy. Free run of the house is for good girls who soil outside, not for bad girls who relieve themselves inside.
Tether her to a reliable human at all times. She is either in her crate or tied to someone whenever she is inside.
This way you can catch her in the act of squatting to pee or poop and you can scream at her (really give it to her so she knows what a bad girl she is, plus she will be startled and stop) and drag her outside for an appropriate potty and praise.
Once you are tied together, if she wants to go elsewhere in the house but you don’t, too bad. She lost her freedom when she made a pile or puddle on your carpet.
This is a pain in the rump but goes a long way toward getting your dog back on track, since she will not be able to soil indoors at will. She will always be caught, punished and corrected.
I am not a huge proponent of stuffing a dog with treats for going potty outside because then they come to expect it and will keep running to the door so they can go outside, squirt an ounce of pee (or life their legs and fake it) and open their mouths for their treat, but if you’ve had success with that, by all means go for it.
It’s not that I’ve never done it and of course you seem experienced so you would know to back off the treats over time so your dog doesn’t become fatter than a pig and you’re not stuck treating for every urination or bowel movement for life.
But I urge you to try the tethering. The first time I did it I was surprised at how quickly the dog caught on that he needed to clean up his act if he wanted to walk freely in the house.
Do not use her regular leash. Either use plain rope or purchase a separate leash that is very different from her regular leash and is JUST for indoor tethering. Be sure to switch leashes for her walks.
You don’t want her to develop a dislike for her regular leash and once you have her on the straight and narrow, you can show her the tethering leash every once in awhile as a reminder of what will happen to her if she ever squats on the carpet again.
If she does ‘backslide’ and soils indoors again, back to tethering.
Behavior modification in dogs is a war of attrition. If the dog errs 1,000 times, you have to correct 1,000 times.
Good luck, hope your girl is soon soiling outside!