I used to have ZWO 30F4 Mini Scope and used it with ASI120MM-S. This camera has a different mounting than the 120MM Mini, but I was able to focus it relatively well. What I noticed for me is that the 30F4 Mini Scope should not be "measured" by the same means as large scopes. It is a very simple scope with just one lens. I even 3D printed a Bahtinov mask for it as I was trying to focus it as best as possible. The mask also shows how many artifacts the scope actually produces. It can be set sharp enough for guiding, but it cannot deliver more than this, IMHO.
Speaking of adapters. I moved over to use ASI290MM mini. It took me almost half day to find out which adapter is it needed to connect to 30F4 Mini Scope. I wish ZWO would do a better job in documentation and provide a consistent naming for threads used. I do not know what is shipped with ASI120MM mini, but with ASI290MM mini, I needed to by this adapter https://www.teleskop-express.de/shop/product_info.php/language/en/info/p2838 - T2 to CS. Inserting the camera as an eyepiece did not sound as a good option to me. BTW, a shaky eyepiece mounting might be the cause for focus problems too, IMHO