Just a few suggestions for smelting arrays:
1. Stone smelting in heated furnaces
2. "insulated heat pipes" - heat pipe that does not loose heat over distance for lategame so there is am ability to efficiently use nuclear reactors in smelting arrays.
3. Electric heater(?)

Thank you for the feedback. I debated these questions when I was making mod.

1. Melting stone: what even is Factorio's stone resource? It can be used to make walls, concrete, and bricks. Sometimes it acts like a rock, chemical, or clay. So I don't know exactly how it should behave.

I don't know if there are any industrial examples of melting stone for building materials so that might not be grounded in reality.

There's a part of me that likes how the fancy furnaces just don't support stone for some reason so you need different setups.

2. Different heat pipes: vanilla nuclear reactors aren't hot enough to melt steel. They only reach 1000 degrees. So it doesn't matter how insulated the pipe is, it won't work. I'd need to make a different reactor entity or raise the max temp on it. Raising the max temp subtly changes the nuclear reactor's balance and I'm not sure how I feel about it.

The built in heat loss is really important for balancing reasons and hard to get right. I tried to strike a magic number that forced mild trade offs while allowing flexibility. The furnaces are OP as it is so I don't want to make them too much stronger.

3. Electric heater: the heat furnaces essentially have built-in speed and efficiency modules. Their recipes are 20% faster, and they consume much less energy, and they pollute less. Having electric heat generators totally negates their only downside: fueling them is annoying.

Factorio doesn't allow heat consumers to pollute, only the generator can. I chose the heat source pollution values very carefully to compensate but it can still be exploited. I'm afraid that having electric heaters would make it even harder to balance (and would require even more art, which to be frank, is a pain in the ass).

I think the late game alternative is to have bots deliver higher value fuel. A single uranium fuel would power a generator for hours. That way you can still get the speed and efficiency boosts without needing more furnace types.

I'm happy to hear your thoughts on this. It's hard to make all the balance decisions in a vacuum.

A bit of adjustment after your answer. Insulated pipes are supposed to be in a late-game, after creation of low-density structures. There is also a place for a tier 3 "furnace" that will allow modules usage. Consumes heat AND power (in proportions like 4:1) but providing faster production rates than electric furnaces. Addition of electric hearths will provide with some new challanges about managing and designing the smelting facility (btw output of hearth is not supposed to be 100%). Also, creating a smelting array by utilizing your mod is harder than you might think and it also requires a lot of resoures thus dealing with the "balancing" issues. And I mean a propper smelting facility. So far I managed to create a single propperly working design that is reliable and "easy to build" (it still requires a shitton of underground belts and inserters). It would be a real joy to see a tier 3 of smelting stuff that utilizes these ideas.

It's not possible to make buildings use multiple power sources using built-in functionality. I managed it with my steam roboports mod but it required a good deal of runtime scripting. You basically need to manually keep track of all the furnaces and loop through them every second. It doesn't scale well into the megabases that tier 3 stuff needs. It would cause a pretty big UPS drain.

An electric hearth with reduced efficiency could balance the convenience of it. In real life you'd lose most of the electricity to transmission so it's not unrealistic.

I'm not gonna lie though, my biggest reason no to do it is the art. Those sprites took forever and I was pretty burned out by the end. The steel furnace's pipes were super annoying. Granted, the electric ones are much simpler.

I'm happy that you found them challenging to optimize (hopefully in a good way). I think finding good patterns is one of the fun puzzles in Factorio and I think using the heat mechanics puts an interesting spin on it.

I'm playing another game of SE with this mod for some reason and I agree that it feels weird that there isn't a late-game version of the heat smelters.