Forecasting what will happen with the solar credit is going to be difficult. I am of the opinion that there is a good chance it will be extended in some fashion, but there are a lot of variables, the biggest of which is the cost to the government with major budget issues right now. So, it might go back to 30% or they might do nothing, or they might just do something like extend the phase-out (so go to 18% in 2022, etc.) (And I think extending the phase-out is actually the most logical thing to do - costs are dropping, and the purpose of the incentive is to help make solar competitive, so it should drop as that happens.) But, I will say that it is extremely unlikely that the 22% for next year disappears, so there is only potential upside in terms of changes to the credit.
However, I am not clear about your guarantee you mention. Is that from Tesla? If so, make sure you have it in writing and also that they agree it applies even if you ask for a 2021 install should they offer a 2020 date. Also, be aware it may take a lot of work to get that check (see the experience of others on the forum - seems like some just got a check but others required repeated calls to Tesla.)
I would probably still try to get it done this year just to be done with the hassle, but if you are comfortable with the guarantee you mention, then it probably isn't a huge deal either way.