Fractional scaling below 100%

I have a laptop with a 14" screen at 1920x1080 resolution and an 17" external monitor with a 1366x768 resolution.
Certain applications don’t scale well on the external monitor: Thunderbird is an example.
I’m happy with having my laptop screen at 100% scaling and would like my external monitor to scale at, say 70%, but this is not possible in Display settings.
I read somewhere that you could modify the ~/.config/monitors.xml file to adjust the scaling manually. I tried doing this, but, instead of getting the intended result, ended up having the laptop screen scaling increased to 125% and the external monitor’s to 100%.
Is there a way to get the intended sub-100% scaling on just the external monitor?

Added fractional-scaling, mixed-dpi

I think if you would scale below 100% on the other monitor you would lose pixels, wouldnt you?

Also, Wayland or X11? You can find that out in the settings.

As a “last resort”, KDE Plasma allows using different scaling out of the box.

I’m using Wayland.
I don’t want to move to KDE: I don’t like it.
For some reason, this issue doesn’t happen with Windows.
That being said, I asked a different issue regarding Thunderbird on different monitors. It is the main programme I cannot use on the external monitor because of the scaling difference.