I can see why you would be concerned, but we can mitigate that by playing defensive, keep our units in our territory and build up until we can expand. In the event that we have to push out, we'll have Thunderhawks, Land Raiders, and Teleporting Terminators as QRF. Also while Astartes can be drowned in numbers, there are weapons like the Multi-Laser, Assault Cannon, Flamer, grenades, Psyker abilities, and Vehicles and Aircraft that can melt the horde too fast to overwhelm us, bring in Reinforcements, or just to evac to safety.
Devastators are for carrying the heavy weapons in case they met a horde or Large creature and Tactical Marines with Bolters aren't good enough. Armories are not only for Chapter relics but also for Astartes Vehicles like the Land Raider, Whirlwind Rocket Arty, Thunderhawks, and their version of the Chimera APC that I forgot the name of. These 2 are meant to be force multipliers which is great to have when you are outnumbered for obvious reasons.
The Cadians are mostly to be a garrison force and to help establish a PDF of regular humans that want to help us but don't want to become Mechanicus or not good enough to be an Astartes or willing to be a serf. Make the most of a resource and I don't know if Serfs make good PDFs but I have 1/2 a CP point so might as well have Cadians. For cavalry, Astartes have their hover bikes and Assault Squads, Scout Marines can also cloak so the need for cavalry as a scout is reduced. If the Mechanicus can launch satellites they become even more unneeded. Plus the Infantry Regiment can be trained to drive IG vehicles made by the Mechanicus so even if it isn't essential, you do have the option later on once a Manufactory is established.
I think I remember QM mentioning that Astartes can learn a language by eating a local's brain and then Psykers could spread that info to other Astartes so, Dialogus is alright but Astartes can do that with BS Gene-seed. So once the Astartes got the language, they can copy that info to serfs to translate with the help of Mechanicus and build a dictionary or just have that downloaded to the regular human officer's brain.
That is a good point but Librarians can handle that and we won't be working with unsanctioned psykers or casters until we build a rapor with the faction they are a part of and by that point the Librarian would have gotten enough of an idea of the situation to act appropriately.
According to the wiki
"They are also notoriously known to quickly summon the aid of the
Sisters of Battle or even the
Ordo Hereticus when they suspected evil from the ranks of their flocks" They only call them when they suspect that there maybe heretics, not before. That means they don't have Hereticus or Sisters in their ranks and had to call them in. Since we are cut off from the Imperium, they'll ask the Astartes to do it and they should be a bit more reasonable since they can just scan a soul and see if they are corrupted. QM also mentioned that Black Templars can show restraint and you can see the Text-To-Speech Podcast about the Black Templars and they are fairly reasonable.
I have no idea who they are, but I do remember that the Imperial Truth was written by Lorgar who turned traitor.