Welcome to higher BR’s. Things will only get more expensive as you climb the ranks (otherwise people would be at rank VIII in a couple weeks in some cases)
The playstyles change drastically as you change BR’s. Now (with the Hellcat especially), you arent meant to go toe to toe with other armor… the M18 really doesnt have armor. You should use its mobility to flex and support on the battlefield (and spot targets). Frontal engagements (especially against heavies) dont pan out well for it without knowing exact weakspots to hit on whatever you are shooting at (unless it is another lightly armored vehicle).
Planes are a whole other breed entirely. Go into ARB at 1.0 and just tinker and learn. You wont jump straight into aircraft and excel unless you are exceptionally gifted and it comes natural, or the other team is so inept that they just fly into your shots willingly. I get my butt handed to me in ARB when I used to do well in it. I went to Sim battles for a bit and god knows I cant fly RB now unless I go back and relearn all of the techniques since it is a different playstyle (much like tanks compared to air).
Angling is only really viable in some vehicles (looking at heavies especially)… even then, once you enter the realm of HEAT / HEATFS / APFSDS it means little to nothing.
That is 100% of your angling problem right now. None of those (except the M4 if you are lucky) are going to do anything while angled…
TLDR: Play as if you have no armor… being seen = dead (which most of your lineup isnt designed to take a hit anyway outside of the M4, which will only bounce poor shots or rounds with just shy of the required pen… which experienced players will still one shot because of the gun port. Another thing to consider, it isnt a race to higher BR’s. Get comfortable with the tanks you use. Use them until you know them like the back of your hand then move up. With a few exceptions, you will still be able to use some from your old lineups (especially the Hellcat) as you climb, and you will be able to wreck higher BR vehicles with it because you will know how to make the tank work for you.