Thanks for the drawing, I wouldn't have understood what you were hoping to do without it.
It's too thick to bend. (Isn't it? I've never tried steam bending before but remember I'm a homegamer)
It's not too thick to bend, but there are some it depends. Species matters, how the wood was originally dried matters, but the main thing is the orientation of the bend and since this leg style would require you to bend the board sideways — across its width, not through its thickness — which I think alone rules it out. It's probably possible but in practice the hurdles for a first-timer might discount this option.
I could resaw it down and do a bent lamination.
If you have the means to do that I think it's your best option of those presented. It's quite a bit of work of course, but mostly repetitive effort and not especially difficult work. In addition to being relatively easy to do it will also result in a leg that you can trust to be immensely strong, and a good result is virtually guaranteed if you get all your ducks in a row before you begin and simply go through the steps methodically.
So this is the winner IMO.
But it'd be easiest —if a little wasteful— to just glue a pile of this together on their 45mm faces and cutting two legs out of the pack.
This won't work. Well it will in the sense that you could glue up the boards as drawn and cut the legs out, but they won't be strong enough.
That last option does mean that the bit of wood that touches the floor, will likely be one or two planks away from the main load. I know yellow glue is good stuff but is this a good idea?
It's not the joints, the glue joints are actually irrelevant (remember that long-grain joints are as strong or stronger than the wood around them). The problem is orienting the grain that way results in short grain at the bottom of the leg, which is inherently weak and is probably guaranteed to split in service. Good grain orientation on legs is important even on things that aren't intended to take any weight, such as a three-legged round table. On a chair it's a safety issue and becomes vital.
If in addition to glueing together as drawn you also laminated face to face, in essence making plywood, that would be strong enough. But it's a lot of work to go to to prep the wood prior to glueing up and you'd have to be very careful to prevent warping after the wood has been prepped to the required thicknesses.