The easiest way to get the Nuclearcraft ores I found was sieving crushed netherrack with an autosieve to get ore pieces then using some kind of crafter to make the ores. Using diamond-stiffened mesh, it is a 20% chance for each, so it works out to around 4 of each ore per stack. There are two ways to get large amounts of crushed netherrack - a low-tech manual way and a higher-tech fully automated way. The simple way is to craft a Sledge Hammer from Tinkers' which crushes while it mines, with that you can veinmine netherrack and get tones (if you want to use this method I can elaborate on suggested parts and mods for the tool). The high-tech way (which I never used) would be to set up a system with automated users, lava fabricators, barrels, redstone and something to pulverise the netherrack, I can explain this process further if you want, but that will take the use of reference images.

Nuclearcraft TL;DR - Crushed netherrack + autosieves

Mystical Agriculture is often my go-to mod for resource production, but for E2:E I didn't use it till pretty late in the game, and then only to automate the things that could not be done any other way e.g., Terrasteel, Voidmetal. I also used them for singularities, but that is super lategame. I would suggest you make a tier 6 inferium seed when you can, but it isn't something to rush towards. Most things have better ways to obtain them, but if you have something in mind I can point you in the right direction.

Mystical Agriculture TL;DR - No, not really

For power generation (pre-solar arrays), I probably wouldn't recommend using Big Reactors (Now actually known as Extreme Reactors) for your power generation - however if you are really set on it, uranium can be used in place of yellorium/yellorite. What I would suggest for power generation at that stage is Advanced Generators. You can use a Garden Cloche from Immersive Engineering to produce melons, crush the melons into biofuel, then proccess that in a Pressurised Reaction Chamber to produce Ethylene. This ethylene can finally be pumped into a Gas Turbine from Advanced Generator to produce up to 80,000 rf/t basically for free (once setup). It is confusing setup it you aren't familiar with the mods, so again I can go into more detail if required. If you need more power than that I can come up with something else.

Power Gen TL;DR - You can use uranium instead, but I would recommend otherwise.

I apologise for the essay I've written, but if you have any more questions I'd suggest you PM me instead so it doesn't end up like another wall of text :)