I actively rebase my kernel on top of rockchip-linux to keep a list of patches somehow compact. My current workflow is not very good for community contribution, still: this is rebase of commit history, but I'm happy to change that.
One thing that is probably unique about my kernel that I release a new kernel version after every: https://gitlab.com/ayufan-repos/rock64/linux-kernel/ and https://github.com/ayufan-rock64/linux-kernel and this is released as debian packages that you can put on any distro and apt-get install: http://deb.ayufan.eu/ayufan-rock64/linux-kernel. In my latest builds, I effectively moved from building kernel each time to installing kernel/u-boot package for simplicity and split of responsibility (single resp repository).
Anyway. I don't want to be a bottleneck, so I would say that the best would be to start a new common repo branch and rebase all our work on this branch, give a few people review/merging capabilities and based the work on the good will of people. I know that being alone and reviewing/accepting is a job that can get boring, as you also need to ensure the quality, splitting the responsibility here is desirable. Now, I do all the stuff there myself, but also I only have to check rock64/rockpro64 which is to be fair not a lot of hurdle for me, but with more dependence on this work we simply need more people.
I somehow like the rebase workflow that we always keep a small number of patches, but I'm fine with merge workflow too