I had my mini split RMA replaced (outdoor unit only) . I had the same concern with pre-charged line set. Therefore I talked with the vendor and made the following arrangements:
Placed order for new Unit and paid for it upfront. It was agreed that once I ship the RMA unit full refund will be issued. And indeed got refund right away once FedEx picked up the RMA unit.
The advantages of such arrangements were:
The new unit came full with cables and pre-charged line set.
I just swapped the RMA unit with new one using existing cables and line set. Tested and all worked.
I might have lost some of the line set contents during the swap ( just isolated the RMA unit from line set first - one might try to isolate the outdoor unit while running - ref to YouTube videos how to collect contents of line set with outdoor unit)
Basically replacement outdoor unit works with the old setup without any issues since then.
Hope that might help your case. But if your indoor unit is included in RMA.... then replacing line set with new one is.... No FUN indeed.
Brgds
K