Hi @Tabby2109, welcome to the forum 
Are you using a library to do this, or forming the messages manually?
Note that the source system ID needs to match the target system, or they may not be able to communicate.
QGroundControl can be quite painful to send custom messages to (because you need to recompile it so it understands them). You mentioned
in which case you may just want STATUSTEXT messages (which are already supported), but if you want to use custom telemetry messages and the like then things might be more complicated, and you may wish to consider using Cockpit instead.
This sensor integration project may be a useful reference
Note that if you’re using BlueOS (or have other sources of MAVLink heartbeats), there may be some extra filtering to do.