It's really great, except for the fact that Browser 2.x becomes hilariously slow to scroll through after around 40 games on the drive. Furthermore the PS2 stores applications as disk partitions, so removing applications creates Mac OS9 style heap fragmentation until you compact everything. Practically speaking this didn't matter at the time, since the PS2 HDD was only ever used for game installs[1] and the firmware was locked to one Sony-blessed model of 40GB HDD.
The NAS route is the only option if you have a Slim PS2. Some of the slim models can be modded to add IDE back in, but not all.
[0] An updated version of the PS2 dashboard - YES, it was updatable - intended to support an online service called Broadband Navigator. It supported e-mail and downloadable demos, including one PS1 game demo that wound up being the basis for the PSP's PS1 support.
[1] Most of which were actually disabled in the US releases
There are PS1 emulators running on the PS2 (one by Sony) but it's not very good.
But you can soft mod a PS2 to run burned PS1 discs. I recently did this because my ffVII disc 2 is scratched.
You can load games from an external drive over the USB port too but the transfer speeds are much slower than the disc drive so most games have issues. MC2SIO recently came out though, which uses the memory card interface to access games on a microsd card - my understanding is that the transfer speeds over that are much better so it's also a good option.
I think you've got this backwards - the only way to run ps1 games from OPL is via POPstarter, which is an emulator. Speed/compatibility is not great for a lot of titles either.
The reason for this is that when you put an actual PS1 disc into a PS2 the system uses the PS1 CPU inside as the main CPU (it's used as an audio co-processor when in PS2 mode), and there's no way for OPL to trigger this while still loading from the ISO in the background.
Though nowadays it's possible to even play games through the SD -> memory card adapter (which is faster than the USB on the system) or even hw mod that enables internal HD on slim PS2.
Would it be possible to just run a Docker container with SMBv1 that only has access to your game ISOs and nothing else? Feels like that'd be better from a safety viewpoint.
Once you get that going, there’s a Bluetooth patch for PS4 controllers, IIRC. :)