Regarding the stockflow plugin, perhaps it would be easier to make it work more like workflow that's tied to a stockpile (having read a separate thread about the difficulty of having stockpile generate jobs in connected workshops)?
Instead of using job manager, stockpile will suspend/resume relevant job in the "Take from" workshops. Then each stockpile can "borrow" the workflow's interface to set up a set number/stacks of items to maintain in the stockpile.
Instead of having stockpile "create" the job, require user to create a job on repeat in the linked workshop. The stockflow UI can indicate if it can detect a relevant job it can control in the linked workshops.
There's an interesting idea in here, but I really don't like most of the details. As it stands, I don't see how this idea improves upon workflow itself. Particularly since it relies on all three of the most complicated parts of workflow.
Stockflow was originally intended as a proof of concept for an idea proposed for inclusion in the base game. That's the only reason the bookkeeper is involved in its default mode. That's also why the UI is limited to one or two lines in the stockpile screen, and an almost exact duplicate of an existing screen.
It also happens to predate workshop links, and you're right that workshop links could be a very interesting way to automatically suspend and resume jobs in a workshop. However, it's much harder than it sounds to determine which jobs are relevant, so I would be more inclined to suspend and resume all jobs in the linked workshop.
So, the core idea, which actually would be simpler than stockflow: A single line on the stockpile screen (tentatively "A: Automate Selected"), when the selected stockpile link points to a workshop, toggles a flag on that link's line (like the R and S flags on workshop jobs). Every so often, each workshop with any flagged links checks those linked stockpiles. If any input stockpiles are empty, or all output stockpiles are full, then all jobs in the workshop are suspended; otherwise, all suspended jobs in the workshop are resumed.
This might cause problems for a job actively in progress when the workshop is checked, which could perhaps be mitigated by running the check every single tick. Similarly, it doesn't prevent workshop jobs from getting cancelled, though checking every single tick could reduce the likelihood.
And therein lies the problem. The point of stockflow is to set up each stockpile once, and never worry about it again. I don't want to deal with jobs disappearing from workshops, which is why I like having the manager create new ones every so often. Then again, flagging links would give me control over which workshop handles which job, at the expense of requiring a separate workshop for each job. (Yes, relevance could eliminate that requirement, but face it, I never got around to determining for certain whether an item on the ground is truly stockpiled correctly, and that's only half of the problem.)
Overall, I don't think I'm going to write such a plugin, and I would be opposed to modifying stockflow to be more like workflow, but if you want to write a third stock management plugin, be my guest.