I think I’ve got this right but it can’t hurt to get a second opinion. I want some lower gears on my 2×10 speed Tiagra-equipped Genesis Vapour. I was toying with going 1×10 but clutch mechs won’t work with Tiagra/any road shifters, as I understand it so I’m thinking 10 speed MTB cassette (11-34) and a 9 speed MTB derailleur (prolly Deore), all from Mr Shimano. The 10-speed bit is in the shifter, no?

I think a new Tiagra 10spd will go to 32t, that should do nicely