+1 to Heather's suggestion. There are ways through Meadow Heights using streets and footpaths with barriers, but if you are happy with Pascoe Vale Rd bike lane, then follow Heather's route to Glenroy. You can continue following streets beside the railway line to Pascoe Vale Station and get onto Moonee Ponds Creek Path. Or, use the new Glenroy Rd shared path to West St and work your way to O'Hea St. (BTW, Cumberland Rd could end up soon with a bike lane/path.) From O'Hea St, use Upfield Path or Sydney Rd or find the informal West Brunswick Shimmy to the Zoo.
Back in Craigieburn, Cimberwood Drive has a bike lane, but crossing straight over to get over to Aitken Creek is a challenge at the best of times. Hanson Rd also has a bike lane, but the intersection with Craigieburn Rd can get busy. Could be better to use the side/back streets from Malcolm Creek and use the pedestrian refuges along Craigieburn Rd on the stretch near the Leisure Centre.
Bridgewater Rd and the main streets that lead to Pascoe Vale Rd can get busy, especially near the school entrances. You might find it easier (at least, safer) to use the back streets with no-through-roads connected by footpaths until you are near Roxburgh Park Shopping Centre.
Mickelham Rd would tend to be out of the way. BTW, it has a good shared path south from Somerton Rd down to Moonee Ponds Creek.
I suggest you avoid the Broadmeadows Valley Path and Moonee Ponds Creek route, especially while the Ring Road works continue on the bridge over the Moonee Ponds Creek. Going down into the valley just means you will have to climb up out of it to detour the Ring Road works. Stay up above, generally following Pascoe Vale Rd.
Psst, do you want to know a secret? Sydney Rd from Craigieburn all the way to the CBD is simple, with no corners (only bends),is generally downhill all the way with no major hills and might be the shortest distance (slightly). But, heck, the traffic is bad. (Understatement of the Century!)
If you like hills, and/or rough joints, then the Galada Tamboore Path (Craigieburn Bypass) is the way to go. I wouldn't recommend the Merri Creek Path for a commute from the Ring Rd south, except for small sections that have been redeveloped.
Here's some contact info for Hume Bicycle User Group (HumeBUG):
Did you attend the Craigieburn Festival on Sunday? The BUG had a stall with bike parking.