There are working end-points and they tend to be stable. If you find a Mullvad server which works with Reddit, you can configure a socks5 proxy for a Firefox container assigned to Reddit (or any domain). This way, Reddit will always use the connection of the working route and your general internet experience isn't affected otherwise. Eg. you can still switch around connections to find a working one for Youtube... Don't forget about this setting, since sometimes a Mullvad server is down temporarily and the container's assigned domains won't resolve (usually enough to count up/down the Mullvad proxy id). This will also prevent you from accessing Reddit without a Mullvad VPN connection.

Socks5 proxy addresses can be found here: https://mullvad.net/en/servers

You need to prefix them with 'socks://'.