nginx configuration no trailing slash / and redirect to trailing slash /
Solution 1:
If I understand you correctly, you just want to use PHPMyAdmin under /phpmyadmin/. Why don't you just deliver (instead of delivering content under /phpmyadmin) a redirect from /phpmyadmin to /phpmyadmin/ and be done with it?
location /phpmyadmin
{
return 303 https://example.com/phpmyadmin/;
}
Untested.