I know things have changed since I last did an RGH setup, which is probably where the confusion comes from; things are changing even though the scene was most active 10+ years ago.
I imagine you don’t need a NAND flasher now that ABadAvatar exists, but I could be mistaken. The process used to be, at a really high level: solder the NAND flasher/dumper, dump the original NAND, solder the RGH timer, then flash a NAND with a good timing profile for that console.

Halo 3 ODST also has cross-platform campaign, so if your friend who was missing it gets it, it should work for everyone.
Edit: Added source for verification.

Halo 2 on Legendary is a pain. I know you said in your OP you thought you might as well have done LASO, but that is 1000x worse, trust me.
I can carry you through the level sometime, if you want. I’ll test the invulnerability glitch on co-op at some point and see how well it works. My guess is it won’t work due to co-op teleporting.

You can make yourself invulnerable on the Oracle level (at least for the last fourth or so of it) by activating the lift to the room where you cut the cables while off the lift, then activating it again so it comes back down, and letting it squish you. It’ll yeet you to the top of the lift and you’ll be invulnerable for the rest of the level. I don’t know how well it works in co-op though.
It was a glitch that originated in the Halo 2 Vista port and carried over to the MCC since they based it off the Vista port.

Sometimes I think I should revisit BioShock. When I played it over a decade ago, I did so on the Xbox 360 because my PC would’ve probably played it at around 15 FPS. I recall the game not feeling optimized for the console, felt tired of the game at the end, and that its gameplay was overall tedious. I wish I enjoyed it like everybody else.
It looks like the number of people who RuneScape (3?) is less than that, but OSRS seems to have a healthy population.

Listen, I love my Steam Deck, but it isn’t a reasonable replacement for heavy Switch users, like this guy and his family. It’s not exactly a high-spec’d machine and as a result, in my experience, none of the Switch games play at their original FPS on the Deck. Some are so bad that they are unplayable. Online play is going to be, at best, limited and far more difficult to manage. Heck, setting it up in general for kids that probably just want to play the damn games is going to be more annoying. The worst part would be listening to, “Dad, why can’t we just buy a Switch?” a thousand times.

Every now and then, I consider using a Google product, and then I remember this.
343 IndustriesHalo StudiosMicrosoft always manages to ruin everything Halo-related, so I don’t have my hopes up.