When I bought my used 2017 I3 Rex in PA, the dealer coded it for me so that the hold state of charge at 75% was available and pressing programmable button 8 turned on hold state of charge if below 75%. I regularly drive 320 miles each way from PA to MA every month (640 miles round trip.
I fill up the 2.4 gallon gas tank (when truly empty, it does take 2.3 to 2.4 gallons to fill). Fill up two about 2.3 gallon cans and put them in the Frunk, and start with 100% user SOC State Of Charge. This is actually about 90% true SOC. As soon as I reach 75% SOC, I press 8, and the REX turns on. I run until it is nearly dry and pull over and add gas before it stops. If you wait for it to run dry, you lose a little SOC each time and eventually you have to stop and charge. When you start back up after filling the tank, the setpoint SOC marker changes to what it is when you press 8 (hold SOC mode), so your setpoint gets lower for each stop. Even stopping before the REX runs dry loses some SOC as the car is programmed NOT to increase the SOC above where you pressed the hold state of charge button. If you drive over 65 or up a mountain, you fall behind on charge and make it up coming down the other side or by driving slower. If you run 80 for an extended period, you slowly lose more and more of your buffer, but the setpoint doesn't move. If you drive slower for the last few miles before filling up you can build back to the point you started with, or close to it. But your mileage will be worse, as your gas is both driving the wheels and charging the battery, so plan on fewer miles per gallon when doing this.
I think BMW did that to optimize fuel economy. You lose efficiency charging and discharging the battery vs directly powering the motor. I just wish there was a way to tell the car to sit there and turn gasoline into battery charge for instances like a power outage where you can't charge and are already at low SOC.
The slow creep down of SOC setpoint could also be fixed if you could refill the tank on the drive (like transferring from a second tank), or just had a bigger tank. A 10 gallon tank would make the i3 a sweet little plug in hybrid that could go all electric for up to 120 miles in summer or 70 in temperatures around 10 F, but you could just hop in and fill the tank and drive from coast to coast like a regular ICE car without having to spend 30 min each 100 miles to charge.
If I am driving home where I have a level 2 charger, I save some gas and finish off on electric because I know I have a charger waiting for me. If I am driving to MA, I do the opposite and try to save electric range and use gas, as I don't know when I will be able to charge next, or if electricity will cost more than gas there. Generally the hotel to office distance is short, and I can last 5 days on a full charge, so getting a partial charge for free even a 110V 10A trickle charge on a regular basis slowly builds the SOC back up during the trip, and I don't need to use gas while there, or while at home, just for the long trips between them.
