I *flushed* my FJR's cooling system *once*. Overkill.
Now, after a few hundred thousand miles, this is my coolant maintenance regimen: Every 30k or so, when I do a valve check I simply drain what coolant I can, and then fill with new. Simple, no?
Unless the FJR has a magical engine that is different than every other engine, I’d imagine this would be just fine. Cars often have 50k intervals... so 30k should be fine even if a little old gets mixed with new. Obviously assuming your gaskets are fine and not blowing-by anything *shrug*
1994 Honda VFR750 custom streetfighter
2015 Yamaha FJR1300A