So far all the mythical upgrade for horses are improving their existing skills, fly longer, run on more terrain.

I suggest that doom should just be able to cycle through the instant acceleration skills without having to drift, Instant Acceleration -> S: Instant Acceleration -> Dark Sprint -> Cycle back to Instant Acceleration -> S: Instant Acceleration -> Dark Sprint

Edit: as a side suggestion, Tier 10 horses should be at least the highest tier 9 stats as a base then build up on top of that, since we are already gluing 2 tier 9s together plus having to go through RNG to awakening then RNG the stats makes no sense.

Overall, the idea of having powerful mounts is a difficult aspect to improve in the world of BDO.

The future of mounts does not look good...

So far both Arduanatt and Diné seem to boost travel speed by improving their existing abilities, so my suggestion would be the perfect idea for the next tier doom horse