Hi,
we have some experience with: Matomo tag manager, Adobe Launch and Tealium (and GTM ofcourse)
You can use Matomo Tag manager without using matomo, but you still have to install the whole matomo on your server and then activate tag manager functionality. This tag manager is quite simple, but functional.
Adobe and tealium age offer much more features, but they cost a lot.
There are many other solutions:
But most of them is just a part of some analytics solution. It would be great to have opensource tag manager.