With a cover on much of the time, mildly high TA levels don't usually cause serious issues. Meanwhile extreme TA levels would require some action. So what you need to do all depends on your test results, which you didn't list.
Borax buffers PH, much as TA does, further slowing PH changes. So it doesn't exactly lock the PH as much as it noticeably slows the rate of change. Borax will also tend to reduce the amount of chlorine you need, which can reduce some of the causes of PH rise (ie the SWG cell will be on less so less aeration).