I can’t speak to the issue of the OTD child, but I am a BT with a secular family that I get along very well with. I think there are two major things to keep in mind:
1) Boundaries… You need to establish that My life is lived this way your life is lived that way. As long as there are clear boundaries it makes life easier. Of course this is easier if you are not living in the same house.
2) Communications, keep them clear and honest. In general people who go OTD do so for reasons, don’t just dismiss them but listen, really listen to your child.
I realize that this might not be popular advice