krzemyk i more see it as a extended support like in Ubuntu LTS
The comparison with Ubuntu TLS is irrelevant, it's something drastically different, GrapheneOS relies on firmware updates, admitting that a device is supported for 5 years, it won't provide full security updates beyond the 5 years, only extended support the time for the user to change device, but will never be able to provide full security updates if the firmware is no longer maintained.
I'm also thinking of acceptable ways other than donations that might suit most, the main problem is when software isn't directly paying, most people don't donate, most users have got used to using free software that stays free and this is the dominant thinking in open-source, poor remuneration of developers explains why the security and confidentiality of a lot of software is so poor, if software isn't financed by a salary or recurring donations, it's financed by the resale of your bandwidth, the personal information you transmit to it, etc. And those who don't do this when they already have no funding end up in difficulty, they abandon the project, someone else creates a fork and the cycle starts all over again.