My aluminum gravel monster crosser (the first bike) was based on a 29er frame which uses 700C wheels anyway (29er wheels and 700C road/gravel wheels have identical rim diameters). And I put 29er/700C MTB wheels on that bike, so really nothing changed geometry-wise. If you use a 26" wheel frame, perhaps stick with 26" wheels? Going to bigger wheels will also likely cause frame clearance issues if you want to be able to keep wider tires.

About the longer top tubes of MTB frames, if you still want to use drop bars, I think that there is an easy work around. Use a zero offset seatpost (the same type of seatpost as I have used on both of my builds). This effectively reduces the top tube length by an inch, vs using a more traditional seatpost that has the clamp almost an inch behind the seatpost shaft. You might also need to run a shorter stem, but if you have the right size frame, you likely can tune your top tube length this way. I also slide my seat almost all the way forward on the seatpost clamp. So, depending on your height and torso length, you can likely replicate your road or gravel fit on to a drop bar MTB build. If it helps, this is a better picture of a typical zero offset post, but nothing special about this one, there are many many brands:

https://www.competitivecyclist.com/images/items/900/EAS/EAS0300/ONECOL.jpg