I don't think that'll achieve your goals, it's far too clunky and inflexible for actually running a business end-to-end. (No knock on Windmill, they're all like this in my experience as the way they model the world is how a programmer thinks about things, not a business owner. BPMN is straight up cancer—avoid.)
I would suggest looking at workflow systems within the VFX space, which they call "pipelines". They are human-driven and maintained, but support very high levels of process automation.
VFX pipelines are written in Python, but the key thing is, they're very good at a few things simultaneously:
(1) Highly irregular work…
(2) That nevertheless requires lots of process, including of the ad hoc variety…
(3) With high human touch (human in the loop), so calendars, task assignments, timelines, approvals, producer dashboards, etc.
(4) That's repeatable on the next project/opportunity, with as much customization as needed.
As a bonus, pipelines handle billions of dollars of work annually under high pressure, with tens of thousands of users, so there's a lot of hard won experience built into them.
I personally have found that there are three levels of VFX pipeline tech that need to be combined and customized to do everything. This is the most recent stack I've set up:
* Kitsu for the producer/manager level, human task assignments. Mainly for managers, has the calendar functionality.
* Prism Pipeline for software integration (i.e. keeping the humans following the process). This is what people actually do.
* Kabaret for compute work. People usually kick off these jobs, but they're handled by the farm. You'll be responsible for everything that happens here. [0]
Don't try to get by with just one or two of the three, even though feature-wise it looks like you might be able to. Incorporating computer-driven workflows is tricky and requires purpose-built libraries in my experience. Just use the right library for the job, and accept that they all have somewhat overlapping functionality.
For most non-VFX work, you'll need to script access to the browser, since that's where people do stuff today (i.e. with the Prism Pipeline library). Launch Chrome, Firefox, or Edge with a debug port and use Playwright as your "plugin" mechanism, with Prism/Python as the driver. You can make your pages do whatever you want and script literally any web-based process, filling in fields from your database as needed, adding data to the page, etc.
For the last six years, I've run a business that does around $5M of annual revenue using this approach, with 4 FTE people (including me). We handle around 40 million compute tasks a day, but also have all of the "daily work" where humans are in the loop. We do all of the tasks of the type you've listed.
HTH!
[0] If you need fully automated task generation (so, no human in the loop), write "pump" scripts in plain Python that push jobs into Kitsu or Kabaret at a particular interval. I personally run them under a god monitor, and have them exit after, say, 15 minutes (or whatever interval makes sense) to get cron-like functionality. This is useful because you can pause execution of any pump process easily, change the interval, restart them, etc. plus you get easy access to the logs for debugging. The scripts themselves are usually less than 100 LOC. I have around 60 of these that run the automatic parts of our business.
Whenever I need custom, temporary behavior in this part of the business, I copy the pump script, edit the original to avoid the special case, and have the new script only handle the special case (with whatever new behavior I need). Done, takes very little time, and is easy to run temporarily (e.g. for the next week, a common occurrence because some downstream vendor has caused a problem we need to work around).
Another approach to trigger tasks/workflows is to have a server that gets pinged by 3rd parties (e.g. Webhooks) and does the same whenever the endpoint is invoked. We have that too, but it's literally just a few scripts. (Most workflow systems seem to believe this is damn near 100% of the stuff that needs to be "automated.")