I believe an easy fix would be to attach the Frostwyrm’s Fury to some other spell. For instance, if the tier set said something like “consuming 15 procs of Rime empowers your next Pillar of Frost, unleashing a Frostwyrm at 100% effectiveness when you press the ability”. This would gives us far more control, and wouldn’t make us more prone to ninja pulling than we already is! This also resolves issues around holding on to your procs. If a big add wave is coming up, you’ll want to avoid consuming your Rime procs to unleash the Frostwyrm on the adds. This will cause detrimental gameplay where you’ll be avoiding a significant part of your rotation to optimise the tier set.
I do feel the weakness of the set in single target must be addressed as well. As it stands, simply holding on to your T29 set will be by far more powerful than swapping to the new one! I believe something more is required that isn’t just the extra Frostwyrm’s. If we continue on the attaching to Pillar of Frost idea, it could increase the buff further whenever it’s empowered for instance. I’m sure the devs can think about a lot of fun and interesting ways to make the set better in single target!
Overall, the Frost Death Knight tier is fun and exciting to use. Unfortunately, it’s incredibly prone to ninja-pulling in Mythic+ and its single target power is far too weak.