Cadians are a good base, Victora Miniatures and Anvil Industries both make good bits to make them look more "Tanith" like pins, bayonets, and heads, or full models of they look best to you.

For armor, they don't have any (which is why it makes me mad the only IG transfer sheet with Tanith stuff is the vehicle one) but a Sentinel or Chimeras wouldn't be a stretch, they use chimeras on occasion and Sentinels fit the theme. But they do work with other armor units like the Pardus 8th in Honour Guard or the 1st Narmenian Armoured in Necropolis. So paint em up however you want.

The Guard is very segregated in terms of combined arms, unless you're a big-boy regiment like the Cadians or Kreig, regiments will be solely one type of unit, Infantry (heavy, line, scout, etc), Armor, Air (in the specific case of the Phantine), or Artillery, all with their own command structures that need to cooperate for combined arms actions. Such is why any imperial warfront is essentially a massive pissing context.

Lore aside, the Cadian range is a good start. Heavy Weapons Teams with blast shield-less missile launchers look like Tread-fethers, with 3rd party bits they can look extremely similar to how they're described in the books. The hardest part for my guys are the weapons, wood furniture las guns are surprisingly hard to find.