The Ultra Short Throw models have reportedly much higher contrast than the standard 4K models at this point. This may be because of some non-specified iris or other mechansim which pumps up the contrast. But, they are reported as being better than normal 4K DLP models.

It may be worth waiting to see how the BenQ 3000i performs, but that's not really a short throw model.

I'm not sure why a drop ceiling would prevent a ceiling mount. Every boardroom in the world pretty much has a drop ceiling and they use ceiling projectors all the time.

CMA440 to the rescue.

The BenQ is certainly 4K capable, but beyond that, it isn't a huge improvement unless you have a ton of 4K content and you have all the sources and equipment in place already for 4K viewing. Resolution remains one of the lower impacting factors for image quality, and you must have very good source content to get the gains of a new projector.

But, it is a solid selection.

It has been a complaint for a number of years now that BenQ doesn't have a direct short throw replacement for the 2150ST, which was a direct replacement for what you have.

The TK700 is more of a gaming model. It is bright, and it is short throw, but there isn't a single 'theater specific' model like the HT3550 in a short throw variant at this time. So, the 700STi is a very solid model, it just may not be a huge step up in quality unless you strictly are going to move to a lot of HDR 4K content.

Remember, 4K content requires all of your equipment to support it.

Would I change screens?

No, I would probably calibrate the projector to the best that it can do on your existing screen before considering any screen change. A grey screen does not improve contrast, it really just makes the entire image dimmer, unless you get one with special optics in it which adds a world of other issues.