Thanks for all your replies! I ended up figuring it out, getting it all the way to 6.60 ME-2.3. I used a long process to get there, but nothing went wrong or anything so I'm fine with that. There might be a more efficient way, as the people above me have said.
Anyways, in case anyone in the future also has a PSP 1000 with 3.300EAR+ and wants to use a confirmed method, this is how I got to 6.60 ME-2.3:
1) I went to 3.71 M33-4, using this guide:
http://pspguides.exelements.net/firmwareparts/Installcustom.html .
2) (basically does nothing much, but I did this for my own good) I went up to 3.90 M33 (didn't bother with the update) so I could use PSP Tool 1.00 (as it doesn't support 3.71 M33) to convert my current battery to a Pandora battery. I did to ensure that if anything went wrong, I could recover. I didn't have any spare batteries. (Although it seems Hellcat, which will be used next does this too, so you could probably make a Pandora battery through there anyways)
3) I flashed 5.00 OFW using Hellcat's, using this guide:
https://gbatemp.net/threads/how-to-softmod-hack-your-psp.293978/ (spoiler for "If you are currently on an older CUSTOM/HACKED firmware...").
4) I then flashed 6.60 OFW using the standard EBOOT file, then went to 6.60 PRO-C2 using this guide:
http://wololo.net/installing-pro-cfw-on-a-psp-1000-psp-phat/ (believe it should be the same for the guide in step 3 though).
5) I then went to 6.60 ME-2.3 using the instructions in the readme in the downloads here:
http://wololo.net/talk/viewtopic.php?t=38203 .
6) Finally, I converted my battery back to normal using PSP Tool again (although I later saw that the recovery could have done it for me). I also set enter to the X button in recovery under the Registry Hack menu (it would seem this is a Korean PSP, and O is set by default to enter).
Yes, this is probably an unnecessarily long process, and I possibly could have gone directly up to 6.60 ME-2.3, but all my googling gave me very mixed results (as no English-speaking forum really had much on 3.300EAR+) so I just combined all the steps I found. I also took the extra step of making a Pandora battery because I had read somewhere someone bricked his PSP by going 3.300EAR+ > 3.71 M33 > 5.xx OFW (forgot which specifically), and was worried about something going wrong. But in the end, it was fine. If any download links are down, let me know by PM or a reply here; I'll have them stored.
I would like to thank everyone who replied to my post, the people who have written the guides, the people who made all the tools/CFWs, and anyone else related to these projects.