The first problem is this strategy relies on you not timing out which has happened to me more times this week in mirror matches that in the entire time since Leviathan was released.
The second problem is that CG was disingenuous (or lied - take your pick) as to the reason behind the change.
"Running Tie Dagger exposes you to a P1 counter"
There were 2 changes to TD: One increased the Max protection gain - this benefits you vs P1 as it makes it easier for TD to survive a mass assist / assists. The second change was to stop TM gain - this has nothing to do with P1, and was geared to stop a Chimaera counter as there is no TM gain anywhere in the default Executor lineup. The second change is the one that is interfering with mirror matches.
As your post suggests, for a mirror match, you need to start with Dagger, Interceptor, MkVI. This runs the risk of timing out and will still leave you vulnerable to a P1 counter, which is apparently what CG wanted to avoid. The alternative is to do a true mirror to keep yourself safe from a P1 counter but then the match is determined by the coin toss as to which Leviathan goes first.
This change is an utter dogs dinner from a game testing point of view.