I love both my Digitakt and my Syntakt.
They’re both beastly workhorses but are also beastly at getting weird in different ways.
That said, since you own the OT, unless you’re looking for a more streamlined workflow, the OT already covers so much of the DT territory and then some.
And since you own an A4, a Syntakt will have a bit of overlap in the Analog voices, where the A4 is certainly superior in that regard.
There’s a lot to be said for the Digi trio’s streamlined workflows, but you already have some Elektron flagships. Were I in your shoes, I’d consider the ARMKII to complete the flagship trio, especially if you like the Elektron way. It is certainly a beastly drum machine (some would argue the best money can buy) and is more complementary to your existing Elektrons.
Just my $0.02