I’ve been giving this a lot more thought lately (and open-source funding models in general). Given timelines (The Zcash Network Upgrade Pipeline - Electric Coin Company) I presume the formal process is to create a ZIP to propose any changes to how block rewards are distributed? (Zcash Improvement Proposals (ZIPs) — Zcash Documentation 5.2.0 documentation). As per that link:
Vetting an idea publicly before going as far as writing a ZIP is meant to save both the potential authors and the wider community time.
Simply, I would propose post-2020 halving (and end of Founders’ Reward) a new development fund is created and given 10% of the block reward. Tentatively, these funds would be deposited to the Foundation who would be responsible for dispersing the funds to develop/support the protocol as needed.
I would expect this to be contentious so having something concrete to debate (and vote on?) rather than hypotheticals is likely beneficial to avoid running into any obstacles of timescales.