I don’t think many people have gone down this particular road since the Electron+Antenna system had to be tested together, so if you change the antenna, you will have to go through testing and certification again. And as you found out, the provided and tested antenna is really very good.
Another approach is a cell site booster. These are often used in large buildings, sports arenas, etc. but also in rural settings. You can get the “home” models for 3G for around US$160. Wilson, weBoost, SureCall are some brand names to search for.