Sounds like what you are looking for is Zentyal - Linux based drop-in for Windows Server AD. It has all of the core features of AD while being a) free, b) stable, c) not a byzantine clusterfuck of half-implemented features that change every two years o_O
Zentyal: it's a debian/Ubuntu based server with everything you are asking for all in one. Great dhcp, openldap, dns, file sharing. You name it. They have a community edition for free, or you can pay for their support service.
Zentyal - install the community edition. Windows "active directory"running on Linux. They have VM appliance and also install on Ubuntu. Everything is done in the Zentyal web interface. Easy peasy.
Only implement the features you need.
Zentyal is free to use as a 'Community' edition.
You have no access to paid support and it does not include some features:
Backup, User authentication in HTTP Proxy, User profile pictures management, IPSec/L2TP, Domain-based
HTTPS web pages block, Importing/exporting of users and groups, Audit mode on/off (file server), Smart
Admin (Kernel manager, Reports & Alerts), Antivirus on-access scan, DHCP Option 150 and 155 for IP
phones configuration. Remote monitoring & management of registered servers (for distributors
Full notes here: https://zentyal.com/community/
I've only seen the 180 day evaluation copies of Windows servers on Microsoft's site, but I haven't thought of looking for anything beyond that.
I have made some research and found you can migrate to cheaper solution, which I will do in my deployments, because this price rise is not form me
easiest, most simple and free https://www.pivpn.io/ A: not so easy if you need multiple daemons or web interface
more complex, but still nice https://zentyal.com/ A: if you are skilled enough, you can use free version without problem A: support multiple daemons on one server with relatively easy configuration A: you can pay for micro licence and have official security updates and support if you want
I have choosen to give a try to free edition and once out key for OVPN Access end we are going to switch to micro perpetual licence which is about 200€ for lifetime licence, no VPN user limits
I'm aware of what Linux, I do admin multiple Linux boxes. But my point still stands for a product that does what AD does to run on Linux is a decade away. No one is even seriously working on it as far as a I know. There is https://zentyal.com/ but that doesn't even remotely come close to what AD can the full Microsoft stack can do.
This is like a replacement for M$ AD, call them and ask why it's not working.
Slow logins are caused by DNS usually, and it sounds like your DNS config is being filtered anyways by the K12 thing - I would start there.
Removing and re-adding the affected server form the domain is also a good idea - because you will likely find your error when you try to re-add and it is easier to solve the problem in that state.
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I still think zentyl community edition is worth a look. It will install on a pretty low end workstation. Give it a test run in your lab environment. https://zentyal.com/community/
I have heard good things about https://chocolatey.org/ for package management.
I never like WSUS for patching workstations. I always relied on a RMM for this. Those get expensive fast though. A lot of people just rely on powershell.
Again not saying you shouldn't get Windows server, just saying if you have the time, try to find a way to avoid it. If you find out the hassle will not be worth the savings pull the trigger on Windows server. I see you already found techsoup so at least you will know you are getting a good deal on pricing.
Theres a few things out there that replicate Domain Services on linux based OS. Cent OS had it last i checked, haven't played with CentOS in forever. The closest experience to Windows Server for AD DC Setup was using Zentyal. I have tested and enjoyed my experience with Zentyal. Give it a try if you want. It ofers pretty much everything you could really need in a Domain controlled environment.