If you decide to encode your material to h264 for avoid transcoding, I absolutely recommend reading about the profiles in this post.
Also, h265 has generally been available on most devices in the past few years, even most low-end ones. If possible, you may want to see if all the devices support it by doing a few test encodes and playing them back.
Lastly, a good rule of thumb: At an equivalent quality level, h264 will have the larger file size, but h265 will encode much slower. So along with compatibility concerns, consider your CPU and storage capacities.
[Full disclosure: I typically use h265 10-bit on everything now, because my playback devices support it. However, I’m considering a separate friends/family folder with specially-encoded copies that are 720p/4mbps/pro-logic-stereo, for both compatibility and bandwidth reasons. Plex’s client will automatically choose the best version based on their device and bandwidth restraints.]
