And I'd agree. Even extending No Mercy to 21s should suffice to make sure you get that Burst Strike->Continuation in whilst NM is active. Adding a second or two of duration to a thing's not outside of SE's wheelhouse, and saying "Hey SE, can you please add 1 second of duration to No Mercy? That way Hypervelocity will land within No Mercy's active duration. Please and thank you" is a thing I can get behind.
But I'd still counter the point of there being too many oGCDs. IDK, I was never too bothered by SB DRK and it's "DA spam" personally, and I didn't feel like it impacted my ability to tank. Did I have to learn how better to time out my cooldowns if I needed to use them in raids and I was going in to a burst moment, or just forego the damage and focus on doing the mechanics the fight required me to do? Sure. I can think of several instances where being a GNB is painful. E2S as I mentioned before it's painful for your 2nd No Mercy. E5S during the "add" phase(s). TEA. But those things can be worked around.
To me, it's not limiting to have one more skill be affected by Continuation. It's not gonna be a fun learning process to be sure, but it's not the end of the world. Being a tank as GNB is already troubling when it comes to using mitigations during heavy weave moments, but I see them more as challenges to solve. "If this mechanic'll happen and I'm about to go into NM, and it'll be unpleasant for me to try and maneuver around or mitigate damage, how can I plot out my skills such that I won't miss out on any GCDs?" That to me says, at it's core, GNB is not a simple to play tank, nor should it be in my opinion! It's technical and requires more understanding of fight timings to maximize it's potential in any given encounter. It's not so demanding if you're just an off tank smacking the boss, but as main tank? Hoo boy, trying to hit the tail end of your cooldowns and prepop them so you don't miss them whilst in NM is pretty fun to figure out.
I'll question how you think a skill that, at [level of upgraded trait to bloodfest]*, will generate 3 cartridges is somehow not going to guarantee you access to your 2-soil spender, outside of recast timing. Especially if 2x Down and No Mercy share a recast timer. But potencies aren't set in stone yet so til then and the people smarter than me in math can plot out "is it worth to always spend a soil on burst strike, or save it for gnashing / 2x down" I still don't see an issue with Hypervelocity, besides the 20s duration of No Mercy.