Yes that’s pretty typical. Some people go in stepwise, some go in all at once. I think you got lucky with your complete recovery, although that is common after about a year for long Covid in particular.

I've had constant brain fog / fatigue for the last 6 months after a brief stomach flu. Got a tip to try the sauna for 3 days a week, which apparently doubles human growth hormone production. After two weeks of this I've finally had 5 days in a row where I don't feel tired. I still need to sleep more than usual, but before I could sleep 10 hours and still feel very tired, now I feel great after 9 hours.

The person who told me this had a patient who lost her sense of smell after Covid, and it wouldn't come back. It came back after a single session in the sauna.

as far as low risk/low cost treatment options to try go, sauna sounds pretty great

Yes, gyms tend to have them here, but we're a very sauna dense country