My advice would be to try transcoding one or two media files first, and test the transcode on different devices. HISTV gives a lot fewer options than Handbrake, but the idea is minimal effort, maximal compatibility.

Specifically, AV1 is a newer standard, and not supported on devices older than ~2020 I think. HEVC (aka x264) produces slightly larger files but works on devices back to 2016 or so, and MP4/H.264 gives yet bigger files but compatibility goes back even further.

For video file size the main things you want to real are the target bitrate and, secondarily, the QP numbers: https://www.w3tutorials.net/blog/what-s-the-difference-with-crf-and-qp-in-ffmpeg/#quantization-parameter-qp-definition--how-it-works

For good quality at a reasonable size you can use the default values of 20/22 but to save a little more space you can probably bump these to 24/26. I went with QP instead of CRF because it's better for streaming (while still giving better perceived quality than a constant bit rate).

As I say, Handbrake is great, does all this and more, but that was my problem with it - the controls look like something out of a space shuttle and I just don't need all that most of the time 😅 I'd love to hear how you find using HISTV vs Handbrake, if you give it a go! 🙌