I have an interest in saying this, but I think AAs sound like a good fit for you.
Style-wise, they’d be a safe bet for sturdy boots on 55 lasts. I suspect all the Spokane makers using that last are running AA eyelets in their main machines. Viberg also uses AAs on many of their more casual “Service Boots”.
AA is a size from the system used by USMC and its subsidiaries back in the day. They’re sometimes referred to as “trade sizes” by other makers offering compatible hardware, like Stimpson. In rough terms, 3s are “extra small”, 2s are “small”, 1s are “medium”, and AAs are “large”.
Some heavy boot makers prefer the even larger OOs (sometimes written 00). I believe Frommer mentioned OOs in his packers book. Personally, they look too wide to me, and often leave noticeable empty space in the holes around even chunky laces. Basically, they remind me of the disproportionate, grommet-sized eyelets from Tandy that I put on my first pair.
You might like a sense of the other options, and not just my conclusions. While White’s Boots mostly sets AAs, I believe they also keep #1s in stock for “dressier” models, like their Stevens. Viberg does the same with #2 eyelets on some styles. Nicks’ fancier “Brandle” line also uses different hardware from their main lines, work and “heritage”, though I’m not sure what it is, and it may be changing. They worked with Marcell Mrsan on those, so I suspect they might be importing hardware.
Long story short, for your niece, “dressy” says smaller eyelets to me, but “chunky” and “Docs” say larger. I’d go with AAs, especially if you think your next lace-up temptation might be more like packer boots than office boots. The AAs look proportionate to me on packers as well as “heritage” boots flying off the shelves these days. They can also accommodate the classic #205 lace hooks, if you ever go that way.
I’d keep in mind that 55 lasts typically come with full metal plates. I believe the ones @Customboots imports do, also. You can obviously pull the plates, but that’s a whole thing.
For the shanks in particular, I think the benefit would be dubious. The 55s have some heel height, for sure, but they’re not the really tall logger or packer heels. In cowboy-boot-speak, they are more like roper heels than riding heels. It’s possible to peg a roper shank, but it doesn’t make the most sense. Ask me how I know.