For me the answer is easy. Genuine first teamers are never going to feature in reserve grade anyway in my opinion.
Therefore leave the under 18s fixtures as they are and play under 19s/20s fixtures on the alternate weekends. The high performing 18s can step up to play alongside the 18s/19s that were kept on from last season.
Why should, for example, Hull FC suffer because some clubs don't want to run a reserves side? Hull have invested heavily in your excellent rising stars programme. Why should that investment be wasted due to small minded clubs trying to save money?
This is a cash issue, not a Covid one. Certain clubs have been trying to pull out of the reserve league since it was reinstated. If they choose not to run one, penalise them.