OK, you have a cheapo controller that only gives max power when you pedal. Nearly all Chinese ebikes used to be like that.
You have two possibilities:
1. Buy a modern controller. For about £40, you can get one from Aliexpress with an LCD and several levels of assist. For a bit less, you can get one with an LED panel and three levels of assist. For about £70, you can get one with an LCD that will make your motor silent and give very smooth and controllable pedal assist. The cheaper options still give a bit of a surge on take off, but not as bad as what you have now.
2. Cruise control. You can easily make a mechanical cruise control by using a thumb lever throttle. Use an old light clamp or similar on the handlebars with a long bolt through it that reaches accross the throttle lever. Do it up just tight enough to hold the throttle in position, then use the brakes to switch on and off the motor.
The throttle works on 5v. Red is 5v, black is ground and white is the signal whire, which gives 3.8v at full throttle and 1.2v at zero throttle. As a safety measure, the controller won't give any power if it sees more than 1.2v at startup until it sees it go down to 1.2v or less.
You can use any resistor array with switches to split the 5v into whatever signal you want to give a fixed speed. You can run it in parallel with the throttle so just splice the three wires. The controller will respond towhatever voltage is in the signal wire, which will be the bigger of the two.
When you use a fixed throttle or cruise control, you can stop with the brakes, which will cut the power, but it's easy to forget and let go of the brake after you've stopped, with obvious consequences. It takes a while to train yourself not to do that.
If you're really clever, you can use an arduino to sense a PAS signal and translate it to a throttle signal.
Apart from the £70 option above, you must bear in mind that throttle signals are for speed control, not power control. The controller has its own algorithms how to translate the speed signal into actual speed. They're normally something like: Give maximum power if your actual speed is below the target speed and reduce it when you get near, then stop power when you reach that speed. The cheaper PAS systems are the same as fixed steped throttle signals using the same power algorithms. The £70 option adjusts the power level for each PAS level independent of speed, which is why it's better.
