Hi mate.
Once you leave the river and go north, there are no moorings anywhere. There are only three options for you being Hillary's, Mindarie and Two Rocks Marinas.
Two Rocks I think would be the cheapest and Hillary's being the dearest if you are in the DPI pens. The club pens are cheaper but you'll have to become a club member.
I was in Mindarie and enjoyed my time there but a lot of a surge would come through. It was not nice when we had weather like what we've had this week. But it was a fun marina to be part of.

BMD, how did you go?

I thought I saw a reply from you earlier but it is not here now so I might've been seeing things..
As llenart has eluded too, is the river an option long term (your original post asked for the Northern suburbs)?

I live in Roleystone and had my boat in Mindarie for a few years, so a fair distance. I worked half way (driving time not distance) and was an easy drive once the Tonkin was upgraded. The river is also a really nice place to be, especially on a 24 footer. One of my most favourite places in WA was moored just off the Mends Street jetty at 6 o'clock in the morning when everything was just starting to wake up. Now I can't fit under the bridges

Don't discount staying on the river....