Yeah, this is a tough problem. I, too, have a lot of customers with pdf signing needs and I can tell you right now there is no perfect answer, but there are some approaches that can make it better.
1. Your second requirement is the most important piece here. If you want the document to stay editable than what you're looking for is "e-signature" not "digital signature". The distinction here is that "e-signature" just refers to putting a squiggle on the page, whereas "digital signature" refers to encoding a certificate from a trusted authority to lock the document. Because you don't want to lock, you don't want digital signature. That makes things easier.
2. e-signatures are nothing fancy at all; just an image layered on top of the pdf. So really you could make that edit with any pdf editor you like, including plain old free Adobe Acrobat desktop. The way to make this easy for users is to change the browser settings in the browser of their choice to by default download PDFs instead of opening them. This will prevent the browser from popping the pdf in a normal (chromium?) interface which doesn't have a Sign button. Instead, once it is downloaded, the user can open it with Acrobat, hit the Sign button, drop a squiggle on the page, save it, and upload it back to SharePoint.
Now the downside: this process works, but it is a bit clunky; you need to download, open in acrobat, save locally, then upload back to SharePoint, which may or may not overwrite the old file depending on the naming and your versioning rules... anyway, I find that this works, but users don't love it. In our implementations we've added our own Acrobat plugin that saves directly back to the SharePoint location where the document was downloaded from, but I wouldn't advise you to try the same (acrobat plugins are NOT easy to make).
Beyond that, there are client libraries for working with PDFs if you want to custom build your own control, but it will not be a small effort; something like that is on my wishlist but I haven't yet found the customer that wants to pay for it.
Good luck!