https://old.ppy.shusing the switch button at all sends you to osu.ppy.sh and does all the wonky stuff you just said, but old.ppy.sh locks you into the old one with no drawbacks
your URL might change to osu.ppy.sh everytime you log in, but you can just change it back to old.ppy.sh and you're set, it won't change anymore
you can also remove the switch button with an adblocker or something of the sort for better experience
haven't seen the new site in months thanks to this