Does anyone know any large companies that allow mega back door Roth contributions for individual 401ks as an S corporation?

Currently hold my individual 401k with Charles Schwab and was just informed that they do not allow this in their accounts…. Have also seen other big players like fidelity also do not have this as a supported option.

Have found several alternatives online which all seem to be small companies I’ve not heard of before. Obviously this makes me a bit hesitant but wanted to see if anyone had any good recommendations