Is there anyone that can extend their knowledge to mechanical and electronic engineering, as well as software? I feel that my breadth in software knowledge is reaching the limits of my mental capacity and I have to shed useless weight now (i.e. I haven't kept up with frontend for 3 years now)

My father was an electronic engineer, and I would love to get into it eventually as I grew up organising his components drawer, burning LEDs and licking solder wire (yeah.. I regret it now), but that's going to be something I'll have time for when I retire.

You have a limited time to learn things. I learn for projects, that keeps my scope limited.

I think I can an extend a little into the non software realm, though my undergrad degree/ first jobs were in civil engineering. I migrated to computers and ended up going back to school for that.

I work writing software for biologists now, though my understanding of the underlying biology in that is shallow (Compared to my biologist coworkers)

Embedded software tends to need at least some EE knowledge, though how much will depend on the job. A tiny bit of ME knowledge also helps (consider thermal management, consider vibrations during shipping, consider vibration in use, etc). For robotics and mechatronics, ME knowledge becomes far more important.

I work close enough to systems engineering to have some insight: The good systems engineers are strong generalists, have one core competency with deep knowledge and expertice, good enough knowledge in related fields to understand and work with experts, and are generally good at engineering.

Complex job, but a fun one! And crucially important for any product more complex than a dish washer.

But please, don't steal the name for some software "engineering" role...

I think parent is refering to the fact that in a formal context (think large Aerospace systems), a Systems Engineer is the role that oversizes all interfaces mech/electric/software and makes sure there are no requirements misalignment.

In that sense the naming has been misappropriated in the SV tech circle to mean a "backend engineer that actually understands what is going on under the hood" sort of role.