Lived in Romania for years and a frequent visitor to Therme, here! Go in the week. Mondays are quiet. Weekends, especially in summer, are hell. I don’t know if it’s still the case, but you needed to be over 16 to access the two adult areas - Palm and Elysium. I used to stick to Palm as not a fan of saunas. The beach area outside in the summer is nice and less busy than inside. The pools with the swim up bars are great, and the food is generally good.
It is super clean, the changing rooms are all private, you have lockers, which you access with a wristband and you also use this to get drinks and food.
Therme stays open late and we would sometimes go for a few hours after work to de stress. The massages are good, but if you want one, book on arrival, as they get booked up quickly.
You can get an Uber or Bolt there, and I’d do this rather than get a regular yellow taxi. That said, Cristaxi are reliable and I never got ripped off with them. You can do the same on the way back, or get a taxi via the machine in the foyer, which gives you the number of your taxi.