In your Task list, click on your computer number.
There will be an option to "Skip VirtualBox VM jobs". Make sure you don't skip them.

To stop receiving the Pythons, go to your computer list on your Rosetta account page ( or copy-paste this link https://boinc.bakerlab.org/rosetta/hosts_user.php ), click on details and click Skip where it says VirtualBox VM jobs.

The reason your other computers aren't receiving standard Rosetta jobs isn't because you haven't installed VirtualBox on them but rather because there simply isn't any work available right now.

How are prospective new users gonna find this stuff out?

That is a very good question. We long-time users have heard about it on the forums, and see the one marked "vbox64" in the applications.
https://boinc.bakerlab.org/rosetta/apps.php

But that is scant information indeed, and tells you nothing about system requirements.
It is a poor way to run a railroad. They won't get much work done that way.

Without seeing what, for example, Milky Way is doing on that machine at the same time it’s impossible to say. You need to look at the full picture, not just one project.

As a example, if one of the other projects has had an off day and fallen behind on its resource share then it will suspend processing on Rosetta, leaving all WUs as Ready to Start, until the other project has caught up.

I would suggest setting your cache to 0 as you are signed up to a dozen projects, almost half of them active.
The smaller the cache, the sooner the system can meet your resource share settings- with that many projects i'd suggest you'd be looking at weeks. With even a small cache, it will take months,
Preferences,
When and how BOINC uses your computer Computing preferences, Computing, Other
Store at least 0.00 days of work
Store up to an additional 0.01 days of work

I would also run the benchmarks on that system- it is showing the default values, and as they are used when it comes to allocating work (as well as allocating Credit for work done) it is probably impacting on what work is done & when.
On the BOINC manager, Tools, Run CPU benchmarks. .

Honestly if you are running 24 hour tasks why even have a cache? Are you running another project besides Rosetta? As long as you have a good internet connection the downtime from finishing a task and getting a fresh one is next to nothing and you are only hitting the server once per day for a new task (vs 3 times a day for 8 hour tasks). All my machines that are set to 24 hour tasks run 0 cache without issue.