@victualsquid This was slightly better on Mastodon 3.x series. There was an option to enter your own instance url and you were forwarded to a follow user popup. There were some security issues with this flow iirc which is why it was changed to current format which is too manual.
I have a solution that would work on desktop browsers. Browser extension is one way as someone already mentioned. I personally use a bookmarklet which works wonderfully.
https://homepages.inf.ed.ac.uk/imurray2/code/mastodon.html
This bookmarklet is great as it works on both profiles AND posts. They are redirected to your instance where you can follow user or like/boost/reply on posts. Last time I checked, Chrome apps did not work on posts.
Create an instance changer bookmarklet here.
Not a solution. But I do not think Vivaldi can do anything about this as they have zero control over the Mastodon software. Gotta live with the issues that exists because of the federated nature of the Mastodon platform.