> [...] if this does not involve installing a whole WB and WHDLoad.

I'm afraid using a "generic WHDLoad slave" requires installing WHDLoad within the emulated environment, which in turn requires installing a WB (unless WHDLoad also runs with just a Kickstart ROM, which I don't know).

> I took a closer look at UAE config and now things are a bit more "speedy"

Make sure to use the JIT version of MorphOS E-UAE (and not the SDL build of the two JIT versions provided in the archive) and to enable the JIT compiler. You need to emulate 68020 or higher for the JIT compiler to come into effect.

> Emulation triggers the fan at max speeds, because CPU is at 100% :(
> So emulating a whole WB.install is maybe not ideal.

From a CPU load perspective, there should be no significant difference between running a program from a WB and running it just from Kickstart. The entire Amiga hardware is emulated either way, which is what's taxing the host CPU.

> I also tried PPaint 7.1 but the UI is totally messed up: [...] lines and
> noise everywhere, just top menubar and requester are displayed properly...

Some old threads with configuration hints:

https://morph.zone/modules/newbb_plus/viewtopic.php?topic_id=3433&forum=16&start=11
https://morph.zone/modules/newbb_plus/viewtopic.php?topic_id=4403&forum=9&start=2
https://morph.zone/modules/newbb_plus/viewtopic.php?topic_id=5084&forum=9&start=1

> forum will not accept 123kb PNG it seems?!

MorphZone generally does not allow upload of attachments, as far as I'm aware. You will have to upload them elsewhere and link from here (or embed in the comment).