1. A PS2 slim can only have a hardrive installed if 7xxxx
700xx (as they have a full network adapter onboard) - 75xxx is the first "Deckard" model with ethernet built into the IOP
whatever I would get will 100% have FMCB installed on the memory card
FMCB, or a swap disk, or a chip - are your options for running homebrew (or more than one; in fact, if you have a ROM 2.30 9xxxx console, you will need one of the other 2 methods to start FMCB)
I cannot directly copy FMCB to another card or it's not a trivial files copy; but once "I'm inside" I can simply use homebrew tools to copy from one card to another
It's true that file managers (wLaunchElf) don't care about copy/move protections - but that's not why you can't copy FMCB (or for that matter any MagicGate executable) to another card; they are individually signed for the serial numbers of the memory card
What you certainly can do once you have homebrew is to properly reinstall FMCB from scratch to any number of (MagicGate compatible) cards!
4. Whatever the PS2 has (I'm not sure what I will find) I can always install OPL to the FMCB memory card, to well, use OPL.
Yes, once you are running FMCB, you can use a file manager and run homebrews from any storage*, and/or reconfigure FMCB to add your favorite homebrews directly to the main menu or hotkeys, as long as those homebrews are stored on memory card or USB
* not all homebrews support being run from all sources; the FMCB installer can in fact run only from the root of an USB drive, while OPL is fairly compatible with at least MC, HDD, and USB supported
5. USB loading via OPL has incompatibility issues (FMVs specially) and stuttering due to USB 1.1 speeds of port.
6. Network loading via OPL has incompatibility issues (not every game supported).
Correct, no single method has 100% compatibility
7. Disk loading on OPL is the only thing to be 100% compatible, although I'm "at mercy" of the optical drive not dying.
OPL doesn't run discs (by itself), the two main disc loading methods are a chip (which supports clean dumps) or ESR (which requires pre-patched dumps, only supports PS2 software, and requires converting CD games to DVD) - both methods have their independent compatibility lists too
8. If optical drive dies it doesn't matter (except for disks loading) since FMCB or OPL will still work fine, FMCB will "bypass" the need for the drive.
Correct, it's 100% optional to have (in fact a "popular" mod for 700xx consoles is to give them a toploading HDD instead of optical drive)
9. If optical drive dies I would have to get a replacement, but I failed to find anything that wasn't just the laser
The laser is the only really consumable part that is not soldered on the motherboard
(console models 5xxxx to 71xxx have the mechacon crash bug, which burnt out quite a few lasers and focus/tracking amplifiers, when the mechacon crashes leaving the focus coils activated at full power, which they aren't designed to be for any number of seconds; this issue is generally associated with hard to read discs, regardless of being originals or not) - but curiously it was fixed very quickly in the following models after class action threats
so, the consoles you want are any 75xxx-77xxx-79xxx plus the old bootrom 9xxxx'es
Protip - check the chassis model (in the bottom right corner of the sticker, you will see some numbers with dashes, one letter, and four letters; the single letter is the chassis - almost every R is a "patched" 9xxxx)