I would only offer reduced rent if they IMMEDIATELY catch up on back rent. Your plans for possibly moving in next year seem flexible, so there is time to have another tenant in there for a year.

My threshold for giving a break is about 1/2 month’s rent. Once they exceed that point it typically does not turn out in your favor to give additional latitude. They are likely paying other creditors who are less forgiving (e.g. credit card companies). You need to be higher up on their list of priorities. You can offer a carrot (reduced rent for the next 4 months if they pay all back rent) a stick (3-day notice) or both (EconProf’s suggestion).

Either way you will lose about a months rent or more, either in vacancy between tenants or in discount the next 4 months to the current tenant.

In any case this is not a long-term solution. You either need to plan on getting new tenants or moving back in as you suggested you might. The predicament of the existing tenants is your short-term problem, treat it as such.