I really like Asbru and have been using it for a couple of years. I used Remmina for a little while but never liked its look and feel.

Not too much active development has been going on lately, sadly, but the latest version still works very well.

https://www.asbru-cm.net/

Seems there is already a feature request open for this issue, please vote for it if it’s relevant for you as well. https://protonmail.uservoice.com/forums/932836-proton-vpn/suggestions/48509147-permanent-mtu-setting-for-wireguard-in-protonvpn

Well, in case anyone is interested, I found a way to workaround my problem.

I edited ../proton/vpn/backend/linux/networkmanager/protocol/wireguard/wireguard.py. Added mtu property to the Config class, then at the end of _set_wireguard_properties added this:

# Added by RandomChain - setting the MTU
logger.info(f'### RandomChain says: setting mtu to {wg_config.ipv4.mtu}')
wireguard_config.set_property(
    NM.SETTING_WIREGUARD_MTU,
    wg_config.ipv4.mtu
)

I know it’s ugly and hacky, but it works for me. I should probably open a bug to be able to properly change it from the app settings.