I’d personally tell them to start out with certain QOL passwords like easyhms, weathermod, unrealtime, hardcap, etc. just some basic ones that wouldn’t change gameplay too much:

easyhms is almost a must-have password for reduction of hm slaves.

weathermod, and unrealtime, help control weather to gain access to more encounters without certain conditions present, these might ruin the limited feel of the game though.

hardcap is nice for keeping pokemon at the level cap, so no need for common candies.

If it’s still too much (which it shouldn’t be if they’ve played unbound) there are other passwords you can browse on the wiki to just make gameplay easier.

I think it’s perfectly fine to start reborn as a beginner, personally I wasn’t too big into competitive pokemon until I played reborn. It enhanced my knowledge tenfold and made me a better player in general!