munki VS reposado

In the non-MDM tool space, look into Munki https://www.munki.org/munki/

this experience is an indication that the opco didn't hire the right expertise. If the numbers you quoted ie 30k desktops, 50k total were not macs, it's clear that the org didn't have the kind of mac experience needed to manage a a new org with all macs.

It's entirely possible for administration of macs.

Start networking/asking around with places like Disney or Pixar, etc which have a large amount of graphic artists using macs. For example, from Disney https://github.com/munki/munki gets you to a certain point.

Other tools like Kandji (lightweight) and the 900 lb gorilla in the industry Jamf gets you the other management / administration bits needed

If this opco that you referred to never got a Jamf rep to work with them and to try out their (yes, very expensive) products, then this is mostly inexperience with the mac ecosystem.

There's a decent open source option: https://github.com/munki/munki

I got to use it at work at Meta (as end user), and it seemed to work quite well. They delivered Android SDKs/IDEs and a bunch of other things that I'd personally install through Brew with Munki.

Munki is not an MDM and I recently learned there's this project called Installomator that might help.

Or Munki

This is a feature of munki called default manifest resolution. Though I prefer to avoid this and explicitly set the client identifier for all devices.

another option is munki https://github.com/munki/munki, for the software update part,

If you’re feeling adventurous, or an archivist looking to keep old macs going, you can run your own local update server via Reposado

I was mirroring all Apple updates going all the way back to Tiger on a server in my house at one point using Reposado. I just turned that off not too long ago and deleted it to free up about 60GB of space.