I read your first post describing your setup and there are a ton of flaws here.
1. The 3DS family consoles can only really handle up to 128GB of storage. The HOME Menu absolutely craps itself if you try to go higher, much less on a
512 GB that I'm surprised you got working at all. The slowdown you saw was literally the hardware crippling to read the SD.
2. SD cards higher than 64GB on a 3DS family console needs to be 64KB allocation unit size, not 32KB.
3. You should be using SD Card Formatter or GUIFormat for this, maybe even both for good measure (easeus sucks):
https://dsi.cfw.guide/sd-card-setup.html
4. R4i Gold 3DS Plus works fine on my end, both DSi and 3DS. That error code looks like it failed to read the cart completely? I'd reinstall TWiLight and try again.
All right, since I am a dumbass who just can't let things go, I kept trying.
First, I would like to thank you for reading my obnoxious posts and taking the time to answer them as well.
That said, your comment regarding the maximum SD card size of the (new)2DSXL being 128 GB can simply not be true. I have never been on a smaller SD than 256 GB (formatted with "shitty" easeus to 32kb allocation unit size) and neither my homemenu nor my custom themes have ever crapped out on me (could be because I keep all my games and apps neatly organized in folders so that the homemenu never had to load more than 30 folders?).
Furthermore, even with my flawed setup, I never had any crashes in literally many hundreds of hours of gameplay. Please believe me, I really appreciate any help and advice, but you basically said in your comment that something, that has been working for me very well for almost two years, can't possibly work. That makes me feel like I am not being taken serious. I am not a native English speaker, and the fact that 80% of all communication is non-verbal (so most of the nuances and context are lost in these forum posts) doesn't make it any easier for me to get what is being said.
I will therefore assume, that you are not gaslighting me and simply have never tried a 256 GB SD in your own (new)3DS system and that you commend is referring to (possibly) older post from people who had such troubles in the past (possibly because they thought a 256/400/512 GB SD for 20 bugs on AliExpress was a real steal?).
I did take your advice of course and double reformatted both my SD cards with SD Card Formatter and GUIFormat (the 64 GB one with 32Kb and the 512 GB with 64Kb allocation unit size) and reading through the forums I also followed TurdPooCharger's guide
https://github.com/astronautlevel2/Anemone3DS/issues/132#issuecomment-360978013 (
https://github.com/astronautlevel2/Anemone3DS/issues/132) to transition Anemone3DS properly from 32kb to 64kb (thanks TurdPooCharger) and am happy to report that everything went smoothly.
The transition to 64Kb did net me a noticeable reduction in overall loading times and smoother frame rates in some 3DS game, so that's a plus and I thank you for that. I did reinstall the TWilight Menu as you suggested and did get it to load (faster too) but the SCFG access/the ability to switch between flashcard SD and console SD appears to be broken for me.
If I follow the guide
https://wiki.ds-homebrew.com/twilightmenu/installing-flashcard I end up in the flashcard TWilight Menu (as if I launched the card with the TWLSlot1LauncherV3) I do get two DSi bootlogos after another (I am quite certain the first is from the TWilight Menu running on the consoles SD and the second is from the TWilight Menu from the flashcard SD) so it appears that the TWilight Menu on the console SD simply launches the card and no longer accesses it.
It could very well be, that me being greedy and upgrading to 512 GB broke it. But since it worked with 400 GB before I can not be certain that the last release (which came out around the same time as I did my upgrade) did not break it for me, since I was able to switch between flashcard SD and console SD even on 512 GB (albeit very slow) and after I installed the new release (I did a complete new installation of the TWilight Menu) it no longer works.
For me that is not much of a problem (I will simply run the TWilight Menu on my console SD for the DSi and DSiWare compatibility and increased clock speed for some games and keep my flashcard as backup for the games that might never run with nds-bootsrap). So I am not writing this because I expect a fix. I fully understand, that someone like me who likes to push things to the extreme can not expect much support when pushing things breaks stuff that works perfectly fine for everyone else who is reasonable. But I like to share my experience with this awesome homebrew because I am having a ton of fun with it and who knows maybe it will work as intended on larger SD cards one day.