The tagspaces pdf-viewer is surprisingly good in several ways (such as the fact that you can highlight keywords for search and simultaneously make edits while the search terms remain highlighted, and other goodies).
However, I noticed first 1, then 2, then 3 crucial things that I am missing for productive work:
The ability to see a list of notes and markings in a given pdf, like you have in Adobe or in Xodo pdf viewer. It is so important to be able to have a list so one can quickly jump to every place where a mark was made in the margin, or an underlining, whatever. Preferably with a small part of its context (at least in case of an underlining) - but just having that list would already be a big relief.
The ‘save’ button (that would save a pdf after edit) is in fact a ‘save as’ button. This must be a small design mistake. It wants to save in the last known path, so one then has to browse for the location of each new document.
There is no eraser for edits. When I reopen a pdf, I cannot remove a mark or underlining. I could rely on another tool for this one. But still. Those are basic functions.
I lately used Microsoft’s Edge a lot, as it is very light-weight, opening pdfs very fast, and editing buttons are ready at hand (just like in tagspaces, except it doesn’t have an explorer that shows up tags of course). Both Edge and tagspaces are easier to use than Xodo or Adobe for edits. And Edge has an eraser. Tagspaces is quickly becoming my preferred tool for reading and managing my pdfs though. But I am really missing those few core functions. Especially the first listed issue. I think this would also increase the value of your product.