I don't know about freshrss - I've used it a few years ago - but miniflux (another self hosted RSS reader) has integrations with a lot of tools, some of which are bookmark managers. I've set up mine with Linkding and it woks just as you described.
Not defending Amazon, but you can do this with Kindle too. I've had my kindles in airplane mode since I bought them (for me and my wife) and I'm adding books directly with Calibre anytime I want. You don't need their whole "ecosystem".
It's usually a good idea to have a line explaining what your plugin does/is for. I have no idea what dap could be and I would lose interest seeing no clear description.
I've have a secrid since 2019 and I'm very satisfied with it. The only issue (on my country's part!) is that our IDs can't fit in it, but I have my drivers' license and that's good enough.
I would love to see something like that. I started self hosting Firefox sync, but it's not working too well and I don't have the time to troubleshoot and test things.
Firefox laughing in 2004 with anything else...