When I had access to a long arm, it was big enough. But I have (rarely) added on borders to a too-small completed/quilted top. You use quilt as you go techniques, and it wasn't a horrible process. In my domestic machine quilting I was mostly using my vintage machine and felt I couldn't be too fancy in the "bed" (center) of the quilt, but I could do relatively fancy work on the borders. I've also brought the backing around to the top for an extra wide "binding" finish. There are some complexities to that, especially at the corners.
I have also combined both long arm and domestic machine quilting on projects for various reasons.
You do such outstanding work! My quilts are bed/use quilts and I'm not so fussy about some things, but getting the stitching to match on different machines is more than using the same thread. Do some test quilting on both machines first. Or recognize that it is going to happen and either decide not to care or make it obvious like a change in thread color or design style.