here's a better class xp code than the thousands of individual ones. Again, can only be used outside of combat. It won't show up in the window until you get into a battle or class change. It'll set class xp to 255 and master on the next combat. The large values get saved though - which probably isn't harmful, but you never know? So for anyone willing to be a guinea pig:
[All Characters - Current Class Max XP]
580F0000 019D76F0
780F0000 0000068C
300E0000 0000001D
620F0000 00000000 000000ff
780F0000 00000230
310E0000
For items, until the save editor is better tested, I've found the best way to set byleth's second item slot. Setting the first slot makes it non-interactable for some reason. If you then empty out the first slot by using/storing it, the second slot item will move up and be sanitized by the game. Repeat to get multiple items. Here're a couple useful codes for this
[Byleth Second Item Slot - Intermediate Seal]
580F0000 019D76F0
780F0000 00000648
640F0000 00000000 000103eb
[Byleth Second Item Slot - Advanced Seal]
580F0000 019D76F0
780F0000 00000648
640F0000 00000000 000103ec
[Byleth Second Item Slot - Master Seal]
580F0000 019D76F0
780F0000 00000648
640F0000 00000000 000103ee
[Byleth Second Item Slot - Dark seal]
580F0000 019D76F0
780F0000 00000648
640F0000 00000000 00010485
Faro's got the item codes in his save editor, so replace the last 2 bytes of the last code with whatever you want. 0x3f8 - 0x400 are the high stat boosting items, for example.