There is a lot more to it than just comparing the numbers that you are, especially if you are changing the fork length… By shortening the Axle-Crown length, you are increasing lowering the front end, and effectively rotating the entire bike around the rear axle pivot point. This lowers the bottom bracket, steepens both the head tube and seat tube, and also increases the reach, as the bottom bracket doesn’t “rotate” as much as the head tube does, as it isn’t as far away on the tangent. This isn’t a huge deal if you’re talking about 10mm of fork length, but if you’re dropping 30mm(to say a rigid 470mm) you’re probably gaining another 7 or 8mm of reach, as well as lowering the stack and steepening the seat tube and head tube(~20mm per 1 degree).
To start with, you’re listing Effective top tube lengths, which aren’t as transferable as Reach measurements, especially when comparing bikes between genres, as the angles can vary a ton.
Secondly, the standover shouldn’t be an issue on a Small Epic…
And regarding gravel suspension forks, see above for the axle to crown effect on the geometry. A 430mm gravel suspension fork would just put the head tube way too steep(72d on a Small Epic) for most people. The MRP is the longest, at 455ish, but only clears a 2.0", which defeats the whole purpose of converting a mountain bike to gravel bike… This doesn’t even then factor all the hassles with running a Boost rear wheel, and non boost 12x100 front wheel…