Annnnd, nope. Still rg lookup error and invisible world. Also not using Optifine, still have the habit of avoiding that combo like the plague from years ago. XD
Given the sheer amount of mods I've currently got running testing each would be a massive chore, I can throw the list here if needed, but I think I'mma go back to BetterFPS for this particular pack. Thanks for suggestion though. 
Greetings all.
So I grabbed an old 1.7.10 pack to test out EE3, never having got the chance to, and after a bunch of messing around to get things to stop crashing and updated, pack loaded just fine. Albeit with medial lag and FPS issues. World loaded fine afaik and I messed with settings so they weren't set to extreme everything.
Went to add fastcraft, and upon reload of the already genned world the blocks were invisible, the only things I could see were floating animals and the sky/void skybox, along with the selection border of anything I was close enough to mouse over. Restarts of the world, the game, and my computer, as well as changing allocated RAM from 4 to 6 hasn't had any noticeable effect, and leaving it alone for a while doesn't make the blocks suddenly appear like it usually does if loading is being silly.
Console doesn't seem to have much that's relevant (adding the pastebin link below) beyond a good chunk of spam in the form of "FastCraft: rg lookup failed".
https://pastebin.com/YiyDBAeF
(If something's missing from it, that's because I set Twitch/Curse to use the native launcher, which meant I had to go in and copy everything by hand. Lemme know if I need to go do that again or find another way of grabbing it that doesn't omit things, or if I should try the jar launcher instead.)
Computer specs are;
Win7, 64bit
16gb RAM
AMD A4-5300 APU 3.40GHz
AMD Radeon HD 7800 graphics card
Drivers were updated about a month ago, and I've been playing Sky Factory 3 for a while just previous to this, so definitely capable of running it.
