Pinboard: Settings -> Backup -> "HTML (legacy Netscape format almost everyone can read)"

Linkding: Settings -> General -> Import "Import bookmarks and tags in the Netscape HTML format. This will execute a sync where new bookmarks are added and existing ones are updated."

The stuff in quotes is verbatim from the admin panels, I have not tried it.

Yes, if you count the export from Pinboard to Bookmark.html and then import to Linkding for once.

One weird thing though is, that Linkding does not have marking a bookmark "read" on first click: This is the most useful feature on Pinboard for me.

Via the pinboard-bookmarklet I pile all the links I come across, then mass read them and can track what's been read because the first click marks them on Pinboard.

Not the one you replied to, but I can check later if I don't forget, but I happily switched to linkding (without importing) and I don't miss anything.