Being such a small kiln, and using a boiler, I assume you will be using hot water as energy source? If this is the case I would make sure that I match the heat exchangers with the kW output of the boiler to be as energy efficient as possible.
I would also keep the conrtrols simple. If I wanted some automatic control, that is not too expensive, I would go for a small PLC with build in HMI screen. You get all-in-one models that come with the basics...±4 Digital inputs (for push buttons and feedback), ± 4 digital outputs (for activating motors, vents)2-4 PT100 / thermocouple inputs....) If you want to go this route, send me a PM, I can reccomend some models
Being so small I would not go for proportional venting, but on/off control...rather adjust the vents so they don't open too big.
Controlling your dry-bulb temperature can easily be achieved by controllling the water temperature from your boiler, by controlling the ID fan from the PLC...let it blow if temperature needs to be increased and shut off at a certain temp. You should be able to keep the DB withing a 2-3 Deg C band.
For humidification you can either go low pressure water spray, or a small steam bath with a heating element like the ones you find in an earn...
Good luck, it is a lot of fun building your own kiln