I don't know if they will target the mets. If they are very close to the prostate they may be zapped at the same time.
If they don't treat them then they will presumably become the site from which further cancer cells will spread.
If they do treat them they really need to be zapped more than once, as they are only giving you 6 fractions I can't see how or when they will do the mets unless they are done at the same time.
If RT starts in September, I think it is most likely you will be fine by Christmas. I think the side effects are proportional to the total dose not the strength of each fraction (unless they gave you all 36 grays in one go, in which case I think you would come out the machine smelling like a bacon sandwich).
Side effects are worse towards the end of RT and a few weeks after, so early November may be when they are at the worst, it could involve fatigue, or minor bleeding from bowels. I wouldn't book an expensive holiday just in case you can't go, but you might want to book a cheap holiday and accept you may lose the money if you do have side effects.
Hope the July holiday goes well pandemic permitting.