is it flat bars or drops? Gear levers from flat bars normally have on the control, but on drops where the gear cable runs under the bar tape on most new bikes, you can use these in the cable run
https://jagwire.com/products/small-parts/inline-adjusters
or similar in the cable stops on the down tube (if you have them, internal cable runs have done away with them on some frames).
cheaper than a new mech and easier than pissing around with a 2mm allen key
Shimano incorporating the cable tension adjustment into the mech seems a poor "upgrade" to me