Considerations for scaling up from 5 to 10 gallons

Your second, second point “2)” talks about getting a pump since it’s cheaper than a kettle with a spigot. Does that mean you’d pump from your existing kettle (which is too small), or does it mean you’re thinking about pumping from a new, larger kettle?
For reference the cheapest hi-temp pump from Williams Brewing is $65.That's considerably more than adding a SS spigot (< $20) to a kettle, which is not very difficult with the proper time and tools. I think that pump is mainly for delivery to the mash. You might need a heavier duty pump for wort.

I’ve considered this 16-gal kettle on Amazon but they don’t mention the gauge of the steel. For the price I’m thinking it’s not all that thick, but it does come with a spigot and a tri-ply bottom.

About lifting the kettle, you could buy/build a stand for your burner to raise the boil kettle spigot higher than the fermentor/s, then let gravity do the work.

No input from me on BIAB other than that's a lot of weight to lift and drain/squeeze without some sort of winch system.

 

I've never done anything bigger than a 5-gallon batch (ehh maybe 6 gallons of something weak like Berliner weisse beer), but here are my thoughts for what they are worth.

First, I bought a Blichmann RipTide pump not too long ago and I really like it. It is very quiet (as pumps go), and it is easy to clean. (I only use it on the hot side, so I never sanitize it, not sure if that would be harder.) One big benefit I've noticed is that if I recirculate my mash for 10 to 15 minutes before transferring it to my kettle, it comes out very clear. Of course, the pump also allows me to move liquids pretty easily, so I can keep my mash tun stationary (which is good because I stupidly bought a stainless steel one, I'm not very happy with it but at least it's feasible to use thanks to my pump).

(As a quick aside, the pump is rather expensive and you'll need some clamps and shit along with it, as well as silicone tubing, so its all-in cost is pretty high and it's definitely not something that fits into everyone's budget. But for people who have room in the budget, I think it's a good purchase.)

Second, I don't quite follow your point about getting the wort from your kettle to your fermenter. Are you fighting against gravity? If so, I don't see how a spigot would help. If not, if you can move the beer by gravity, then why not just use a sanitized auto-siphon? That's what I've been doing as long as I've been brewing (except for my first batch, when I just dumped the wort through a strainer) and it works fine. A little slow, but it also has the nice feature of allowing me to leave a lot of trub in the kettle.

 

Get the pump, or build a tall 3 tier stand.

You will spend more time heating and cooling. Your burners and chiller need to be up to the task, or if more time is not a problem keep what you have.