You are right Arsenal's weakness is in the second tier players when Arsenal schedule is tight and one of the players is injured then problems can occur so Arteta needs to focus more on problems like this. This season Arsenal have brought in 3 players Midfielder, Striker and Defender now their strength is almost equal but I'm not sure if the current squad is enough to go through all the games I think Arsenal need a few more players. Arsenal are reportedly planning to bring in a midfielder , Mohammed Kudus is on the list but it looks like Arsenal do not have enough funds.
Those are the series of weaknesses in Arsenal squad last season that Arteta is trying to cover for this transfer window. If they had players that were almost on par with the main squad last season, they might be able to keep up with the game and be consistent until the end of the season. Looking at the Arsenal transfer market this time will make it easier for Arteta to rotate players when dealing with a busy match schedule and anticipating injured players.
I think this transfer step will bring Arsenal to the level of success next season, although on the other hand the dominance of Man City, Liverpool, Man United and Chelsea will make their steps difficult.