The Bibbulmun Track Foundation
https://www.bibbulmuntrack.org.au/ is a very useful place to start. The website is a wealth of information itself. The office is staffed by very friendly and helpful people - usually staffed by "older folks". It's worth dropping in if you have the time, otherwise phone.
They did have a small publication on suggested 2 and 3 day walks. I don't know if it's still in print.
Yes, single overnighters are not that easy to come by. It's a long track in a big state!
New guide books have been published, but I don't know how recent the maps are. Maps clearly show access and parking points.
While the Kalamunda to Mundaring Weir walk does look short, it's a handy one to start as a bit of a "shake down" trip. Especially if you're new to the Aussie bush. It provides easy access, plenty of roads not far away if you need assistance. Not to mention a great pub for lunch at the end. A handy incentive for anyone collecting you!)
Another option is from Mundaring Weir or Perth Hills Discovery Centre out to Helena and return. Ball Creek is not a very popular stop with many walkers.
Then there's Brookton Hwy to Canning and return. Or to Mt Dale and return (or pickup at Mt Dale).
Or Sullivan Rock to Monadnocks, return via Herold Rd.
Overnight parking is an issue with both Brookton Hwy and Sullivan Rock. Vandalism is unfortunately a frequent problem. You're best to arrange drop-offs and pickups.
Nth Bannister into White Horse Hills, and return via forest tracks is also a nice trip.
I hope that gives you something to start with.