If you still struggle with pH rise at 50 TA, you could add borates to attempt to stabilize pH some, but be sure that TA is 50-60 before adding borates. You can buy pH balanced borate solutions such as Gentle Spa to add borates easily. A cheaper but much more of a hassle method of adding borates is using borax and... acid. Because of the low volume of spas, it's not that expensive to add borates as a pH balanced product. Pools on the other hand, it's much more expensive to add pH balanced borates, so in pools people tend to add borates with the borax/acid method.