The problem with 64kb intros is not the availability of 64gb of ram, it's that the level of work for something really impressive requires you to have a really advanced toolset, and even then there are stumbling blocks.
The stuff I've written for 64k intros:
- Rendering system (software renderers in the 90s and latest a non-RTX raytracing systems with global illumination), these alone is kinda what people write or wrote MSc thesis works on.
- SDF based effects/rendering (this is what 4k intros often focus on apart from music, here it becomes an addition to the above toolset).
- Often some keyframing/skinning systems, either customized or imported from Blender(now) or in the past 3dsmax, lightwave,etc. (and matching the keyframing maths of the tools), the kind of shit that made game industry companies adopt 3rd party engines because it's hard to get correct (luckily for demos with small teams we can avoid some pitfalls of the formats).
- A modular softsynth (with sequenceing, genrerators and filters) with a MDCT based compressor (think MP3 like compression)
- Texture and image via generators and filters.
- A few generations of visual editing tools to have visual editing of various things listed above.
(Yes, some people have created more abstract things in 64k intros but those don't push the envelope as much imho).
Oh and once all that is done, nobody will run it because...
- The exe compressors used to make things small have been adopted by malware writers and antivirus programs will flag everything because antivirus software just profiles the unpacker routines.
(4k writers have less of an issue with this because the 64k compressors are more drop-in whilst crinkler that's used for 4k's is a custom linker that will have less of a profile and requires custom build steps).
As for Raspberry PI's, there has been compos to target them and phones but the capability of a Raspberry PI and phones is close enough to a PC that you're not really constrained in any pracitcal way apart from needing to push it onto that machine.
On that tangent however Pico-8 and similar pseudoconsoles have been popular since they often provide a simple IDE environment that let's people get straight to it and have a clear recgonizable restriction level.