This is more of a physics problem than an engineering problem. Garmin InReach uses an omnidirectional antenna on the Iridium network. With an omni antenna your

This is more of a physics problem than an engineering problem. Garmin InReach uses an omnidirectional antenna on the Iridium network. With an omni antenna your S/N ratio is simply too low to send bits fast. If you made the device powerful enough to roast birds in flight it might work for faster uplink but then it would be dangerous to you too and the FCC would never let you use it anyway. And even this wouldn't improve your downlink speed. The only way to improve the speed in both directions is to use a directional antenna, which points most of the uplink energy toward the satellite and concentrates the satellite's incoming energy to get the S/N ratio high in both directions. That's exactly what Starlink does. This is made even more difficult because we're talking about LEO satellites and LEO satellites move. So the antenna has to track the satellite. Again this is what Starlink is doing. (You can also use GEO satellites which don't move, but then latency becomes terrible. That's how the older satellite internet systems work.) It's certainly possible to build a portable, directional, tracking LEO satellite antenna. That's exactly what the Starlink RV solution is. But it's never going to be as small as your Garmin unit.