All good points, although I'd say dwarven would see more use in Vikaastea specifically. The Tridanarr of the Oaken Shield looks like a dwarf in his picture and even the ruler of the city is half-dwarven. Vikaastea seems lousy with dwarves and gnomes, and from a party optimization standpoint, Tamara has no choice but to take elven being that she's a half-elf so we seem to have that covered.
And it's absolutely true that deep speech will likely only come up in
ad hoc scenarios engineered to require it solely because you took it, but I'm not sure if Ornux is the type of DM to purposely play to the party's strengths like that. I just thought it'd be a cool, thematic pick for a Great Old One warlock specifically given its description as "the oldest language" and "the language of aberrations originating in The Far Realm". It struck me as the language that a Great Old One would use, if it used any.
[ +- ] Deep Speech
Deep Speech was created by the Aboleths, so its the oldest language. Its the sound of stars forming in the void and collapsing, of galaxies swirling. Its the sound of a glaciers relentless advance, the roar of an earthquake.
Deep Speech was the language of aberrations, an alien form of communication originating in the Far Realm. When written by mortals it used the Espruar script, as it was first transcribed by the drow due to frequent contact between the two groups stemming from living in relatively close proximity within the Underdark.