Mounting & dismounting
The obvious solution is a well-equipped step-through. You could build a new bike around a step-through frame – either off-the-peg such as the Thorn Sherpa or custom-built – using components that give you the required gearing range. Or you could buy a “ladies’” touring bike such as the Dawes Galaxy Cromo Ladies.
The editor suggests trying a dropper seatpost, as used in mountain biking. A dropper seatpost descends rather like a variable-height office chair, then returns to its original height at the push of a button. It’s used for riding downhill on a mountain bike but for you could move the saddle out of the way until you are aboard and riding.
Cycle’s Technical Editor