On long trips on bikes without cruise control I use a combination of a throttle lock and a cramp buster (lets you hold the throttle with the heel of you palm). Not really good for following traffic, but I don’t.
Throttle lock, sets a minimum throttle speed. https://www.amazon.com/Go-Cruise-GC-A1BK-Universal-Throttle/dp/B00ERXG4TM
Cramp buster, they come in multiple sizes, get one that fits; https://www.amazon.com/Crampbuster-CB4-Throttle-Mounted-Motorcycle/dp/B001L9KI56
I have also used a nylon gadget that puts a speed bump on the throttle grip, it works almost as well as the cramp buster. I found a knockoff, no reviews but it costs a $1.19 Shipping is $1.99 https://www.amazon.com/Superjiuex-Comfortable-Anti-Slip-Motorcycle-Universal/dp/B09VGP5PR7