Personally, I actually find Adobe Analytics easier to implement than GA/GTM (I manage both for our company).
There are two ways to do this.. one, is to choose one tag manager and do both implementations from there (I would choose Adobe Launch / Adobe Experience Platform Data Collection over GTM if I had to choose one - it's much more powerful, and I will add some more notes on this later), or you can use both in parallel, keep your GTM for Google Analytics, and use Launch for Adobe Analytics.
Given you already have GTM.. I would suggest doing the parallel route.
Adobe Analytics is much more powerful than GA, there are a lot more customization that you can do for your dimensions, so start small with the basics, but as you grow more comfortable with the tool and what it can do, I think you will quickly outgrow what you can currently do in GA.
I would also suggest starting from scratch on your Adobe... this will take a little longer, but it will be a good way to learn the system, without limiting yourself to GTM, but of course, your will likely want to keep your basics close to GTM as well.. but this in itself is a good review for your site... what is working, what isn't, what can you expand or drop...
@frederikwerner did an awesome video on a simple 10 min Adobe Analytics implementation:
https://www.youtube.com/watch?v=VRi3WdwkA3g
Now, before, I mentioned that Adobe Launch was more powerful... there are a lot more triggers both built-in and fully customization that you can use..
One of my major issues with GTM is when I create a click element trigger, I have to specifically code all the node levels to pick up clicks on nested elements (div, image, text, span, etc) on some of our fancy "button" links... in Adobe, I only have to reference the parent element and all clicks on the children are captured....
Doing custom code triggers is so much easier in Adobe too... in GTM, you have to use a trigger, to fire a tag, then that tag does custom code, so that you can create a second trigger for that, then fire the tag you want... In Adobe Launch, it's just one custom code trigger and your tag.
There's lots of help here.. if you know how to use GTM, you should have no issues learning Adobe Launch.. and if you run into any snags, myself, or other people here will glad to help you out.
Good Luck!