Honestly, getting past A+ is a huge pain. I'm not terrible at the game but an unfortunate series of events saw me lose my S+ in Splat Zones and I was careless in S, and managed to de-rank to A+... from there, I seriously went all the way down to A- due to all the randomness, before climbing up again. Once you reach S, the matches are more balanced, and S+ is fairly reasonable. The higher ranks of S+ feel very balanced. X is kind of scary.
My tips would be: set yourself a limit whereby you'll stop if you, say, get two cracks in a session. Also, if you really dislike the players you are with, step out of the lobby for a while and try to get matched with another group. Pay attention to the power level of the game. If you're matched against people who are obviously better, well, you get serious rewards if you beat them and the meter is not really affected if you lose, so what do you prefer - fight it out with all your might or throw the dice again for another set of players? Sometimes consistency is nice, even if it feels one-sided.
As for weapons, some are better for some modes. What are you swapping between? It's useful to understand and make the most of whatever range you have, and that's hard to do if you keep changing them. Pay attention to equipment, too. Does the stage have lots of non-inkable walkways? Run Speed Up. Does the mode mean being caught in enemy ink lots? Ink Resistance Up. Etc. Vary your clothes based on the individual stages and if you die, think, what caused that? Lack of ink? Ink Saver Main. Etc.
Finally, it might sound silly but actually set yourself a goal during the game and carry it out, and vary it if the circumstances change. If you want to achieve one specific thing, it helps you focus. But you can get mired in a losing situation if you don't adapt.
Keep going and good luck! I remember the feeling when I finally reached S+ for the first time. Then they put X there just to tease me ;-) But I made it.