This is a wonderful application for taking hand-written notes digitally. It supports (from what I can tell) tons of graphics tablets and I certainly never had a problem with my devices using it. Check it out!
While working as a TA at my university in Winter 2020 I used XournalPP to correct and grade submissions and I noticed that it lacked a layer merging functionality. In summer 2021 I finally got around to adding it and while it took me a while to properly update the UI for it, thanks to some helpful feedback from the projects contributors/maintainers I ended up very happy with the changes I got to make to the project.
They are already included on master and should get included in release 1.2.