pyr0ball

The stepper drivers run directly of the main power supply through Vcc, so assuming you haven't added any extra heaters or other crazy amounts of power draw, you should be just fine adding another motor.

Using the second z for stability would be a more traditional use of those parts but your idea sounds like a lot of fun!

You can't both bridge your current router and use it for WiFi AP because of the hierarchy of the network. You'll need either a compatible AP-capable WiFi radio in your be router (hard to find, limited compatibility, I think you might top out at WiFi 4e (ac1350) in terms of what hardware is available.

Alternatively, you can use a dedicated access point alongside your new router which is easier and will get you better, more up to date, technology

Get yourself a decent router capable of running OpenWRT, which will allow you to set up vlan's for your lab, and (I would also recommend) another separate vlan for your IoT and other "smart" devices.

The TP-Link Archer C7 is old but reliable and has a lot of open source support.

If you're feeling more adventurous You could also build your own router with any computer that's got two or more Ethernet ports using PFSense, Firewall-NG, or IPFire