munki VS munkireport-php

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,

MunkiReport (Munki clients report here) https://github.com/munkireport/munkireport-php

I'm not sure that is going to do what you want, but then again I have no SCCM experience with Macs. Since you have a small number of Macs and your company isn't willing to spend money on new services, I think you might want to look at Munki. It's an open source tool to distribute and run PKG installers. It is often set up on a Linux system. So if you have a VM infrastructure, you should be able to set this up for free. This article should get you started. If you find this to your liking, you can also add some tools to give a graphical interface to Munki, such as Munkireport-php.