Another option to consider is going with friction shifters so you don’t have to deal with matching the correct cable pull.

More expensive but my personal favorite on flat bar conversions is using bar end shifters paired with paul thumbie adapters

https://www.paulcomp.com/shop/components/drivetrain/shifting/shimanothumbies/

Someone else mentioned brake levers. I have enjoyed the shimano sora levers which have an adjustable tab to make them compatible with short or long pull (thus good for mtb or road style brakes) https://www.amazon.com/SHIMANO-Handlebar-Bicycle-Brake-Lever/dp/B01D8CNM1K