Loving all the ideas you have!
I think I will start with - yes to a tiered system. But also not too much of a tiering system for all power sources.
So as a base, I guess coal is generic, but on a galactic scale, I can’t find it making much sense? Coal is the product of millions of years of compression of carbon based life forms if I’m correct?
So depending on your plans for what different planets will be like, depends on the whether you want coal to be your basis for power, or to be something that’s there, but not considered frequent or good in any way.
In an ideal realty I believe using hydrogen generators, (using hydrogen as the fuel source. This could be extracted from particular planets atmospheres or seperated from water on the torus, etc.) Hydrogen generators are realistic and with an abundance of hydrogen in the universe, it makes sense as a basic fuel source that can be gathered from everywhere. And in the even of a hydrogen generator being shot. Boom. It makes sense as far as I can tell.
Moving on, you definitely want solar power. Whether you use the sails already in the game and modify them so they generate power, and do a logarithmic curve as you said in terms of power generation, on how many sails are placed next to each other and such, or you add a new type of solar panel, one type in brick form, for fancy energy efficient designs, and one in a flat block form. Solar power wouldn’t require so much of a fuel source so much as it light require coal/carbon found semi rarely throughout the galaxy to craft.
On top of the fuel source, I believe different stars should provide different amounts of power. Main sequence producing standard amounts, blue giants producing overloading amounts and such. I can’t quite remember all the types of stars and their effects on things. Hopefully you understand the general idea though?
Next up I believe you would want nuclear/fission power. Uranium, will be all across the galaxy as planets will be radioactive balls of magma for a while, and it also means adding in the step of finding the uranium in asteroid belts, and planets in the future. On top of that, separating the uranium 238, from the 235 (I think they’re the right numbers?) then after having them processed, there can be multiple uses for the two parts, nuclear missile, fission reactor fuel etc. the fission reactors could come in compact form, but having some form of multi block structure allowing for massive amounts of power generation would make sense. For larger ships, and small stations perhaps?
Next up I like the idea of Fusion power! Fusion power requiring a multi blocked structure again, using electro magnets to contain the plasma created, and then however you want to use the plasma as fuel. It could be for super heating water like a fission reactor, or it could channel the plasma into some form of superconducting plasma generator of sorts, another seperate generator block(?) with the fusion reactor being the creation method of the plasma.
To create the plasma I’m thinking it be logical deauterium Be the fuel source. It is commonly referred to as the method in science fiction, as to how fusion energy is possible. But deauterium would be a very time consuming form of fuel to create requiring machines, time and plenty of power. As well as an amount of H2O.
I’m not 100% sure where you want to go next, but I definitely agree with the end game power source!
I think it should be some form of singularity generator, harnessing and using the forces(technically not a force) of gravity. This would be a serious construct, built only in the largest of installations, capable of powering entire planets without a worry. It would require some sort of black hole in the centre, being stabilised (?) by up to 6 lasers. The lasers would fire bursts of energy into the singularity preventing expansive growth, whilst the gravitational tidal forces are harnessed. Being around this could also suck you into it? (Wacky gravity of sorts) the more lasers firing the better control you have over the singularity. If the lasers stop firing, the singularity gains control and tears the power containment sphere apart. If the lasers fire too much, you could over control the singularity and close it. Meaning you must re open it using another form of high tiered power. So it would be a matter of finding a balance of maximum power generation, and safety.
Of course most of these systems could be basically implemented without requiring many other things. But as a base point of access into the power system, it might work.
In terms of harnessing gasses to create other chemical compounds, whether it be oxygen, H2O, carbon, hydrogen, etc, it could be cool to either find a gas giant with the particular gasses in its atmosphere, or for making almost any kind of gas you need, go to a nebula, harness the gasses there, and using machines, convert the gasses into whatever you need. This would mean much more time and power required, but ultimately mean gas making space stations in the nebulas.
Hope that covers fuels well, and I guess it covers a bit about the energy topic too? But hopefully the suggestions help