I need to run electrical from the right side of my home where the panel is located on the inside wall of my garage to the total opposite left side of the home where I have a 115v mini split to wire up using 12/2 romex. The easiest way would be for me to come off the panel on the exterior right side of the house and run conduit strait up and pop in the attic then across to the opposite side of the house and pop out again coming down with conduit to a disconnect for the unit. Can I do that whole run from the panel with romex going through the conduit on exterior but when I enter through the attic can I keep going with romex wire without using conduit until I come out on the opposite left side of the house then continue down to the disconnect with conduit? Conduit run on each side would be about 15ft.
No, because NM-B is not allowed outdoors.
The outdoor conduit counts as an outdoor location.
Other issues you'll run into
NM being inside conduit is not a problem, however it takes a great deal of cross-section so you need to use fairly large conduit.
NM exiting conduit in a protected location and continuing onward is not a problem (presuming of course that NM is legal at all in your community; it's not in Chicagoland for instance).
If the conduit is longer than 2 feet, there are thermal derates which limit you to four circuits of 15-20A per conduit. Does not apply to <2’ long conduits. Larger circuits are limited more severely.
Thus the idea of "1 fat conduit coming into the top fitting with all my Romex in it" does not work.
Outdoor panels are a less desirable choice anyway, since it puts your costly and sensitive AFCI and GFCI breakers outdoors.