'Lightweight' encoder for high bitrate video, 60 FPS, YUV444?

[Selur]

If retail 1080p Blu-ray discs come with MPEG2 streams at 20-30 Mb/s and they look the way they look, I would assume a 1360x768 source should do fine with a worse, but more lightweight algorithm than H.264.

retail mpeg-2 uses 4:2:0 so 12bit of raw information.
4:4:4 is 16bit data. So just sticking with 16bit increases the data that needs to be saved by 1/3.
Additionally, more than doubling the amount of pixels and increasing the frame rate also requires more data.
-> so be aware that lowering the options of the encoder might work against your resulting file size.

Or, better said, what are the most complex features of H.264 a high bitrate video would not exactly need (excluding QP)?

The complexity of the format doesn't really change.
What is your goal in lowering complexity?
Getting faster encoding or faster decoding, or something else?

Ryzen 5 5500 + Geforce 980, so hardware encoding isn't that interesting. (Hardware encoding without b-frames is often worse than using x264 with ultrafast preset.)
-> Have you tried x264 and x265 in their ultrafast presets and adjusting quality through a lower/higher crf value?

Cu Selur

Ps.: ProRes and other lossless formats that support 4:4:4 will be nowhere near mpeg-2 or H.264 compression ratios.

FFMPEG's MPEG2 seems not to

Naturally, since even high profile in MPEG-2 only supports 4:2:2.