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I don't usually use GB5 but I just got about the same results as you: 774 & 1703. I'm not sure there is much to say or conclude about those benchmarks. Here's what I obtained with GB4.4.4: Catalina/Clover: 3705, 7500 Big Sur/OpenCore: 3809, 7401 Big Sur/Clover: 3821, 7460 Strange about those graphics glitches you encounter in Mojave and Catalina. Not seen/experienced those. Did you have the Capri framebuffer memory patch in place? Anyway, water under the bridge if you're happy with Big Sur and it's fully functional with Clover

That statement in Dortania's documentation about OpenCore not needing a DSDT is a little confusing and misleading. Possibly something lost in translation as one may say... All bootloaders universally pull/read ACPI tables from BIOS (i.e. firmware). On paper, no Hackintosh bootloader needs a (patched) DSDT. It's just that over the years, ACPI patching developed greatly and what was done years ago through DSDT patching is nowadays done through dedicated SSDT patches that leave the loaded DSDT table more or less untouched. OpenCore came out at a time where such development had reached maturity, having already been vastly experimented and successfully used with Clover. That's for the story (or history)... Focusing back on the E6230, I detailed the patches implemented back in 2018 to A19's original DSDT in my E6230 BIOS settings thread, so you may indeed rework them out through SSDTs or pre-programmed patches of the bootloader. It wouldn't be too difficult, most of them are well known and documented. The only patch missing in that list for the brightness keys and details are available here, in my E7250 guide or here, in my 7490 guide (same patches applied throughout). Jake Lo has also published SSDT alternatives for this brightness keys patch. I was (and remain) too lazy to jump back into that and revisit, especially as Catalina or Big Sur work perfectly with my patched DSDT (even with OpenCore, Yes siree!). No issues and no worry to have on the matter.

Go back to the installer main screen and open up Disk utility to reformat your target partition HFS+, i.e. Mac OS Extended (Journaled). It can't be a new volume on an existing container, that only supports APFS format, it'll have to be a separate partition. Adjust your disk arrangement as required. Please note that, as stated in my guide, I reverted to Clover on what was an existing Big Sur installation made with OpenCore. I'm not 100% certain that you'll actually manage to complete a fresh Big Sur installation with Clover 5128 or r5129 (latter is also Ok to use and I've actually updated from r5128 to r5129). But give it a try, you'll soon find out. You say this is a fresh installation but you've actually downloaded Big Sur and initiated its installation from Mojave, not from a booted USB installer, haven't you?