I think this is the crux of it.
It isn’t particularly technologically complex to build tooling to generate, simulate and post it on chain as an executable. We shouldn’t need to verify calldata because the code that generates it should also submit it. There would be a single source of truth - the blockchain.
This post (SPP2 Stream Implementation - Preparing the Executable Proposal) from @5pence.eth links to a repo we were playing about with that achieves two of those three puzzle pieces in the context of that executable - it generates calldata and creates Tenderly simulations. It would not be that much work to have it submit the executable. It would not be that much work to add a user interface to this.
There are a number of posts about Governance Tooling, and funding. For example: Programmatic Tooling Rewards: A Proposal for Sustainable Governance Infrastructure
Similarly there are numerous fantastic Governance tooling providers, many of whom applied for the Service Provider Program. But… if they don’t do what we actually need, we should just pay someone to do it properly. Just saying.
Given that @blockful got funding to do Governance stuff, they should probably do it…