A place to share alternatives to popular online services that can be self-hosted without giving up privacy or locking you into a service you don't control.
For Example
We welcome posts that include suggestions for good self-hosted alternatives to popular online services, how they are better, or how they give back control of your data. Also include hints and tips for less technical readers.
Useful Lists
G2 has a list of all the software products in this category, ranked by easiest to use, etc...
Can compare the products side by side, depending on how many endpoints you have, you may be able to pull this off for free or very little
Many of these are cloud/agent based, which you will want if you do not plan on maintaining VPN to each site. And are designed for MSP and multi site layouts, so yes, grouping together by organisation is possible in most.
You could take a look at n-able or Datto. Both are common software for MSP's here in the Netherlands. Both are also licensed so will cost some money but got everything from monitoring, AV and patch management
We use Datto RMM at our MSP. Definitely not open source or free. But it has what your are asking for and will keep up with your company as you grow, without needing to migrate to another platform in the future.
Ninja or Datto are two great options widely used for MSPs.
We've been using Datto RMM since 2016 and it's been pretty solid for us. It has a friendly interface so it works well with customer interactions. It's not open source, but at least for us it's been worth the cost.