Snoke literally doesn't bat an eye at the destruction of starkiller base and we are shown a huge army stationed there. There is no drastic change in strength. You are literally arguing that because we are not shown a fleet in TFA that the best assumption the audience should've made is that it doesn't exist.
Hux seems to care, he goes and runs to tell Snoke about it, kind of hard to tell what snoke's feelings are supposed to be since he's a weird cgi hologram monster. (Also this is before Hux is turned into a complete joke)
At the end of TLJ the remaining rebels including Leia are laughing and smiling even though everyone just died, so I guess if in the opening of TROS the rebels essentially take over the galaxy with a giant military that comes out of nowhere in another week that would be cool. Every movie we can just have the control of the galaxy switch back and forth in the opening with the use of previously unseen forces!
"The FO, aka the giant army that had the resources to decimate an entire system of planets, with the destruction of their base not making them bat an eye, ehhhhh, these guys probably don't also have a fleet, and thus the part where the characters aren't surprised by their fleet is a contradiction due to this conclusion I am making."
It just makes absolutely zero sense to reach that conclusion.
In TFA the only planet they shown to control is Star Killer Base, 1 planet in a story that takes place in an entire galaxy. Hux and Kylo are always on the same ship #2 and #3 in the FO have to share a ship the entire movie not really showing power. That really doesn't speak to them being that powerful. Kinda implies the opposite, that they've been spending their time and resources perfecting their super weapon since its their only chance to win. The resistance which is made up of like 30 X wings in the movie is enough to be a huge thorn in their side.
Again Hux cares and it seems to be Hux's project more then anything. Snoke is more concerned about Luke and Kylo but not concerned enough to actually send more troops to help find that map throughout the movie even though they are busy doing nothing apparently.
You are asking why they didn't do things that would lead to open war
in the middle of a cold war?
Haven't read any star wars books/comics yet, ain't gonna start now to try and make sense of these movies.
The whole joke in both TFA and TLJ is no one cares about Jakku hardly the kind of place a galactic war is gonna star over and anyways Kylo/Hux are already there with 1 ship you know a couple more wouldn't hurt.
The FO already blew up the Republic before they attack the planet Maz is on so... kinda late to be worrying about open war. Again sending a couple more ships seems like a good idea.
Guarding their secret home base with the massive fleet is again something that doesn't make sense to start an open war about. Why would guarding Star Killer Base start a war but not the fleet just chilling wherever it is during TFA? And even after they destroy the Republic at the end of the film they aren't really shown to be guarding it, once again kind of late to be worrying about a cold war when they just nuked 5 planets.
