Hello,
I am wondering, that you have problems to get in focus with the DMK 21.
I have a LUNT 60 Ha and combined this with my DBK31 without any focus problems.
I think, the difference between a lunt50/DMK21 and a lunt60/DBK31 is not so great, so I think, it should work.
I also use a ASI120MM with my lunt and never had problems.
I hope, this will help you in your further decisions...
Joachim