Move steam tank out of artillery, would not be fair. But what i would love; is to pass the canon from steam tank as an option.maybe the main problem is that you have to choose between steamtank and a serious set of artillery.
In that case we could take steamtank out of the armoury section (would that be a problem?), and just make a rule that it also count as a cannon regarding the 0-2 limit. It could count towards auxiliaries as to avoid the most extreme gunline plus steamtank lists.
Then we could have a simple steam tank without canon oriented fight, a bit cheaper, and outside from artillery.
Then we should just adapt the rule of +1D3 if not shoot.
Maybe something like a reserve of 3 or 4 steam power. And each time you want speed up and add a D3 or use a breath attack you use one steam power. (max one use per turn)