What alternatives to Stab-Lok breakers are there?
UBI is a 'replacement' stab-lok sold under various names.  Chinese based company.
2/3 failed to trip below the 135% rated current.

Siemens/Murray looks about 2/3 passed at or very closely above.

Eaton, Square D and GE had no test failures.

This is NOT a huge test.  



Second image is from testing 3000 breakers that were removed when renovations were done.  Breakers had not been abused. (Wet, building hit by lightning, etc) and were all in service at the time of removal.  

Based on this, I will go with a Square D sub panel and breakers.

Images is from the article: https://inspectapedia.com/electric/Circuit_Breaker_Failures.php