If South Africa can score 220+, and take a lead of 120+, I think it will be a good collection for them. Because we know this pitch in Mirpur is spin bowling friendly. Bangladeshi players could not perform well in the first innings. There is a possibility that the Bangladeshi batsmen will be able to bat well in the second innings. Less because there is no batsman in the Bangladesh squad who can bat responsibly and play big innings. Bangladesh lost both Test matches badly as we saw against India. And I am sure Bangladesh team will not be able to win the test match against South Africa.
If we talk about Bangladesh so they didn't play well because they made only 106 runs with all team out. But like you said if SriLankan team will go above 220 runs so that will be a good lead according to the recent performance of Bangladesh. But we can't underestimate them because they can make a good comeback in the second inning. Let's see what they both will do?