Yeah, I think you're right about the range. I've been thinking about the balance since I woke up and there's really no way around the fact that it's just too strong as it stands. I do find myself thinking that a straight nerf might be a little too much though, so I'll add a setting for it so that players can customise their experience - maybe have a default 0.8 multiplier of the vanilla max range but let players change it anywhere between 0.5 and 1.
More kinds of shells is an interesting idea but I'm not sure it's the right choice to include them in this mod. Maybe a separate one with a selection of incendiary, fragmentation, high explosive, poison, perhaps nuclear shells, is a good idea; and I could add basic variants of those as well (except perhaps for the nuclear shells) with this mod as an optional dependency. A guided kinetic shell might work in this context too, reintroducing the radar into the recipe and eliminating the accuracy penalty for it.
I agree, limiting the lifespan of the gun was a bad idea. Chalk it up to having been awake less than 20 minutes, I suppose.
Direct fire mode, while an interesting idea, would require an entirely custom targeting script. I don't even know if it's possible to alter the targeting behaviour, truth be told, but even if I can it'd have to be a ground-up rebuild. I'm not saying no, but it is something I'd need to look into more and then decide if it's worth the requisite effort. Just on the face of it though, it seems like it should be its own thing (e.g. a mortar turret) rather than a secondary fire mode of an artillery turret.
A faster rate of fire, or perhaps a cluster shell to achieve a higher effective rate of fire without making the gun shoot faster, is also an interesting idea. If the latter were chosen it could be folded into the hypothetical additional shells mod mentioned above. Perhaps for the former, adding a separate research to improve the basic artillery with something like a rapid-reload magazine that lets it fire a few shells in rapid succession followed by a rest period might be interesting too.
I don't think I'd want to make any major graphical changes like resizing it. I'm not entirely sure how to go about that with the turret sprites themselves (as opposed to the turret/wagon bases) and it's not really something I want to sink too much effort into as I'm much more interested in the gameplay aspect of artillery.
Thanks for all the suggestions though. Do let me know what you think of my interpretation of your suggestions, feel free to tell me I'm being an obstinate fool and not listening, etc