Which new road bikes can have seat height 890mm or more?

Sounds like you're searching for a carbon bike, but a steel bike might be more likely to be available in your size. The Surly straggler has a 620mm seat tube (62cm) option, and the Disc Trucker has a 640mm seat tube (64 cm) option. Steel bikes often have horizontal (not sloped) top tubes, so you end up with much less seat post exposed. They also have round tubes so you can more easily find a longer seat post if need be. The extra weight isn't much assuming you spec it out with the right components for everything else. As most you'll end up with a couple extra pounds which won't be a huge percentage of your total weight (bike + rider).

Realistically - you're a tall rider, several standard deviations away from "average" And that's a difficult place to find gear that fits.

Simply raising the saddle and lengthening stems doesn't do the whole job, because the bike's wheelbase doesn't increase as much.

Shorter riders can sometimes compromise using kids bikes, (which isn't meant in a bad way) whereas us taller people have fewer options.

I'm 195cm tall total, and my preferred BB-to-saddletop is 840mm.
I have a variety of frames, but the best fitting ones are around 58cm. If I could find affordable frames in 60-66 cm I'd buy to compare.

You might have to consider a custom made frame to fit your parameters, and they are significantly expensive.

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Check out Conor Dunne's bikes, at 6 foot 9 inches / 206 cm he's a long lad.

That's an enormously long seatpost on a tall frame, and notice the long head tube too. As such he presents a lot of frontal area and gets less benefit from the draught, but provides a lot of shelter for anyone behind him.

(Conor's the one in the middle)

Upshot - tall people bikes make seated-climbing harder because once the grade exceeds 3-5% then weight on the saddle is subtracting from your front wheel weight, leading to skittish handling and lack of control. A good power stroke on a steep climb will wheelie easily for a tall rider with a long seat post, so you're out of the saddle sooner/longer than an average rider, and short riders can stay seated for almost all climbs.

Solution - look for a frame with longer chainstays, to help keep the weight forward. This gives a frame with a longer wheelbase and better climbing/straight line stability at the cost of less nimble manoevering. IE a race bike tends to be twitchy, and a longer wheelbase calms it down somewhat.

LINKS: https://www.stickybottle.com/uncategorized/video-all-records-are-broken-to-make-6-9-irish-rider-the-biggest-bike-in-the-pro-peloton/