I've used a teleprinter to decode the cuneform. Why would you need to run relay from a reference. Is it a voltage regulator you are wanting instead?
You are correct. In any set up like this the transistor has to offload (12-5)/0.2 or 1.5 W give or take.
A tin can 2N2222 can barely mange 500mW So you do need a beefy transistor and very likely a heatsink.
Adding a very basic current limit is straightforward. The current through sense resistor R6 creates a voltage drop, this turns on Q2 which robs Q1 of base drive.Imax is very roughly 0.7/R6.
You need to limit the output current from the opamp with R1 else the current limit transistor will be shorting it out.
You can offload circuit self protection using an LM317L:
www.radiolocman.com/shem/schematics.html?di=644555 Note that emitter follower circuits like this have a tendecy to burst in to oscillation.
The precsion of the reference is being lost in the opamp and feedback resistors.
You need a good precsion opamp and low temco feedback resistors to get the set point and drift where you want it. Those will not be cheap.
Super cheap. Ditch the opamp.Still dissipates 1.5W. Works great and is easy to tweak.
Another way if you have a 3 terminal series ref.
Buffer a 5V reference. The reference's own precsion feedback resistors and opamp take care of the output voltage set point.
You can set a current limit (R2) to stop your transistor melting. R3 sets a minimum load current.
It still has to shed the same amount of heat.