I do something similar to what you describe: import into LR on MacBook Pro, cull, tag, develop, etc. on the MBP, then move to the big catalog on the PC. Things I've learned along the way:
- I started by copying the big catalog from the PC to the Mac. Just the catalog, mind you, not the pictures or previews. Then I deleted all the pictures from the copy of the catalog on the Mac. Why? That left me with my very large keyword hierarchy. If you have a complicated keyword hierarchy then you want to make sure they're in sync across the two catalogs, otherwise things wont "line up" when moving images from the Mac to PC. To be clear, I would have been happy with exporting my keyword hierarchy from PC and importing it to Mac. But I didn't immediately see how to do that, and frankly catalogs are small: 25000 images and my catalog is ~330MB, it's just faster to do what I did than it is to screw around in menus looking for an option I wasn't sure existed.
- When done culling/taging/developing on the Mac, I select the images and "Export as Catalog" to the Mac's drive. Then I copy them over the network from Mac to PC. On the PC I can then start LR with the big catalog, do "Import from Catalog". Easy peasy. I don't export everything from Mac to PC: I usually have other shoots in progress on the Mac that still aren't ready to be exported.
- I'm not time-constrained on the copy mentioned above. I don't have situations where the images on the Mac must be moved to the PC now. I do it when the delay doesn't matter--usually end of day.
- Why do you think yo won't have room for the catalog on the Mac's SSD? What you haven't mentioned is how many images you expect to have in the Mac catalog. Probably the most I've ever had is a couple thousand (all raw). Space was not a problem. Of course, it depends on what else you're doing with the laptop. But catalogs themselves are small, and I'm assuming you're talking about having some (relatively) small amount of images on it.
Also, if you use the wired network and run 1Gbps ethernet then that is going to be a faster way to copy than using external drives: regardless of using USB3, the actual platters on mechanical drives aren't going to let you push data on/off the drive that fast. Unless maybe you use an external SSD drive. Also, using external drives implies a multi-step process: Mount the drive on the Mac, export to there, unmount it, mount the drive on the PC, copy, unmount it. Over the network you're just copying in one step, without the mount, unmount, mount unmount steps that also add time.
And if you find that you're not time-constrained enough to need the wired ethernet then it's even easier to do the copy over WiFi.
You're going to get a huge benefit from doing most of your work on the Mac's SSD. I've found that when I'm done editing on the Mac that everything after that is not time-constrained. You might discover the same thing. I would start simple. Don't try to go for mega-fast expensive external transfer drives until you prove to yourself that you actually need it. And if you need it you'll make sure you appropriately solve problems that you really have.