How to compromise between byte rate, resolution and fps
Since bandwidth is fixed, first step to maximize quality is to full use that bandwidth so bitrate should be set to that number.

Frame rate will make a modest difference, once a fixed bitrate is specified, since typically x264 places keyframes 250 frames apart and so all or most frames in a random second are delta frames, so leave that between 24 to 30.

For keeping a given quality, size increases, but not linearly, with resolution. So 1080p, with 2.25 times the no. of pixels as 720p, takes around, say, 1.5 to 1.8 times to keep the same quality. This choice depends entirely on the nature of content. For live-action material, 480p is good enough. Remember, till the advent of HD a decade ago, American television audiences had been watching 480i feeds for decades. For 2D animation, with sharp edges, 720p may be a better choice.