Max-Zorin Google has promised five years of firmware support for phones on sale now, so even if they cancel the upcoming 7a et seq. there's a long time to figure something out. Meanwhile I suspect the GrapheneOS team has plenty of near-term things to handle, e.g., new phones, plus also a Pixel tablet on the way.
The team has publicly expressed willingness to work with OEMs focused on security and has negotiated some with at least one OEM.
I suspect at present this isn't an area where advance planning is constructive or even possible.