The ST-6700 should be 10 speed rear.
Are you getting 9 clicks? 9 shifts + the outermost position = 10 speed.

For the front, are you counting the 'trim' clicks?
The shifts should be similar to the St-6600/6603. 5 clicks for 3x, 3 clicks for 2x.

lol I should have caught that 9 clicks means 10 gears. I’ve done a little reading about these components and have come across trim, but know nothing of it. I suppose it’s a 2x10 then. The brifters really need to be torn down and cleaned, but seem to be in good shape otherwise.

The 'trim' feature just gives you 2 positions for the FD in each ring. Shimano's labeling of which position is considered to be the 'trim' varies with shifter, but the idea is the same.

See below for an example.

Looks like 11 speed chains have the same interior spacing as 10 speed, so an 11 speed chainring should work fine.

My rear deraileur has a max capacity of 29t and a max front difference of 14t. That means an 11-25 cassette and 53-39 chainrings… at least that’s the only off the shelf combo I found, and only in 105.

Do y’all think I should do that, or get an Ultegra 52/36 or 50/34, and get an extra chainring to reduce the difference?

I think 50/34 will be fine as is.

Yeah the old rear derailleurs suffered from too low capacity and too small cog size. Anyways, you can switch to RD6700, which has a total capacity of 33T and max cog size of 28T , and matches the rest of groupset to boot. You can then run 11-28 cassette with any of these cranksets you've mentioned. If you are riding on flatlands, 53-39 is totally fine. If in the hills then 50/34 is better.

Back to the original question of what crank, I’d consider going a bit outside the box for a 50/34 crankset. New Albion is a good looking, forged crankset. No bonded spiders like modern Shimano, so a plus in my book. The square taper is fine, and a good sealed BB will last many miles without trouble.

I think I’ll go for a standard 50/34 or similar and just live with the couple illegal shifts. I’ll have to check my freehub for cassette options. It’s possible that it’s a 10 speed with spacer which would make it compatible with 11 speed cassettes. If I end up needing a new deraileur, I’d probably go ahead and make that upgrade.

11 speed would require a complete new shift setup. Different shifter, FD & RD.

Shimano changed the 'pull' ratios for 11 speed, so FD & RDs are not compatible between 10 & 11 for most road groups.

yes, but having that availability in the hub just reduces friction of the upgrade. I suppose it’s just $50 sunk into a 10s cassette if I decide a later upgrade. If I sink $100 into a cassette and long cage deraileur, I’d probably spring for it. Just thinking out loud.

If you go with a 50/34 crank, an 11-25 cassette will give you a reasonable low gear. Probably enough for non-mountainous riding. Or an 11-28 cassette if you stay away from small/smaller combos that require more chain take-up.

You can run the gear inch calculations and compare it to what you have on your other bikes.

I currently have a hybrid type bike with 48/38/x and 11-30. With the kid trailer I never go below 38-20, and alone it’s around 48-17…. It’s all flat around here. Thinking a 50/34 and 12-25 will do nicely. I’m leaning towards spending the extra $150 for all new 105 kit since the current shifters need covers, cassette needs a spacer, and probably will run into a few bucks here and there that will make me regret trying to salvage the current setup. I read the st-6700 is (was never) the greatest and even modern Tiagra is an upgrade.

If you're not going to do any racing (crits or road races), then you can select 12-25 or 12-30 Ultegra 10-speed cassettes. Only drawback is that they are $65 and $75 respectively. The 105 11-25 one is $38 at universal cycles. Its not great that 105 only has one option, but it is what it is. Maybe there are some other sources for 105 10-speed cassettes.

2-piece, hollow spindle cranks, IMO, are the magic combo of good and old.
- 5x110mm BCD satifies my retrogrouch desires in looks, and can go as small as 34T inner ring. If you want a lower-low, get a triple with 5x74mm inner.
- External bearings are backwardly compatible to BSA shells, yet place the bearings closer to the crank arms, so reduces radial load, and fit more balls, so are more durable and smoother than internal cartridges, and can adjust to take up bearing slack (restore preload) after the races burnish in, and that makes the bearings last many times longer. Mine came with ISO-External bearing spline pattern, the most common, but it was only slightly more to buy a 4-way bearing wrench, so I'm set for the future on most standards.
- Crank can come off the bike on the road without a special crank tool, just an allen wrench.
- Low (road) Q-factor; lateral pedal spacing.

The engineering trinity of lighter than air, stronger than steel, cheaper than dirt. And looks good. Just hard to find in silver like the old days. Here's mine, $60 with rings and bearings, mine came with steel rings, was supposed to be aluminum rings with lift pins, they sent me the correct rings when I pointed that out, and updated their ad.