It isn't about the commonality of the bug, but the level of access it gets you on the type or massive scale of the target. This bug you your blog? Who cares. This bug on Discord or AWS? Much more attractive and lucrative.
How would you make money from this? Most likely via phishing. Not exactly a zero-click RCE.
Yes, but this is not a particularly high access level bug.
Depending on the target, it's possible that the most damage you could do with this bug is a phishing attack where the user is presented a fake sign-in form (on a sketchy url)
I think $4k is a fair amount, I've done hackerone bounties too and we got less than that years ago for a twitter reflected xss