To Power Me, first off I am new here so take my opinion with several grains of salt because I may be way off base but here is what I think on this matter.
If you really break it down there are many different elements to riding a bike so that it is both safe and enjoyable. All of them take practice. Some of them for you will be easy and intuitive but those same elements may be more difficult for others and require practice before they can perform them as well as you do. To me, your concern, starting off without being able to sit on the seat is really, starting off standing on the pedals. I think you need to practice riding (not just starting off) while standing on the pedals and you should really do this on a conventional non-ebike. This is an important technique to be comfortable with so you can use your weight directly over the pedals to increase your pedaling power when on a steep hill, it is also essential for riding over some very rough, pot hole strewn road surface as well as negotiating an object in the road that you see at the last second and must ride over it while trying to maintain control. Standing on the pedals gives your body a built in shock absorber that is much better than any suspension you can purchase for a bike and being able to absorb road rough road hazards increases your ability to maintain control. Once you have practiced and gained some confidence riding while standing on the pedals, your current concern of "starting off while standing on the pedals" will seem like a trivial fear you won't believe you ever obsessed over. Pedaling while standing up is different than starting off while standing up but by practicing the pedaling and standing while moving (at both very slow speeds and medium speeds) you will gain the skills and confidence that will greatly aid in learning how to start off, on the pedals. I would STRONGLY suggest mastering these techniques first on a non e-bike. Once you've got it down, then introduce the next new variable, the pedal-assist power on the ebike and the learning curve will be much easier and your level of fear will be greatly reduced due to the skill you learned on the non-ebike. One of the games/competitions we used to play as kids was to see how slow you could ride and still maintain balance. No one could win this game while sitting on the seat, everyone was standing on their pedals because you can more easily and quickly shift your weight to the left or right as you begin to lose balance/tip in the opposite direction. From a control standpoint, at low speed, I think you have a greater chance of balancing your bike while standing as opposed to having the majority of your body weight on the seat. So start out pedaling and stay off the seat for 4 or 5 (or more) rotations until you get some momentum going which gives you more stability. Once you have a little bit of speed and stability, then make your transition to the seated position. Right now it seems like you want badly to get on the seat as soon as possible and that should not be the goal. Starting off while achieving and maintaining a comfortable level of stability should be the goal, and once that is achieved, then transition to the seat. Hope this helps.