Tessa is ranked #229, which isn't really that popular. If it was one of the top 20 or even the top 100, I can understand being put off by the popularity but it isn't ridiculously popular. If
Tessa is your favorite name, then go for it.
Tirzah is usually pronounced TIR-zə, so most people are not going to guess that it's TEER-sah unless you correct them incessantly.
Tessa goes a lot better with
Hattie and Myer.
Tirzah is more Old Testament while your kids names sound more modern.