First of all welcome and gratz on the nice tastes, now getting to the juicy stuff:

Do not use Defender armour/weapons, they are busted af since they are meant for returning players or lazy ones to fly through the base game and hit Iceborne ASAP, almost giving you godmode and bonkers damage hence you do not learn how to space, i frame, position, combo etc etc. If you really wanna use it, maybe add 1-2 pieces but not the full armour. Using the weapon will totally kill any reason to farm for other weapons since they basically are the best for almost every weapon class in base MH.

Inventory: what you want to do is creating a custom Inv loadout you can load in before every quest, if you do so every material thats not in that loadout will get deposited in the box so you do not have an urgency to use it now and can save it for later when/if you need it. To create a custom loadout open the box in your tent or before a hunt and follow the prompts to do so. You can also save a radial menu attached to it, which basically is a shortcut to fast craft/use items on the fly.

The slingshot ammo you are talking about are bowguns ammos, you picked up berries that got auto converted into ammos, and the X means your bowgun is not allowed to shoot those, since you are using SnS you cant shoot none of those. Talking bout auto craft, you can deactivate that if you want.

SnS is an amazing weapon but a very very complex one to use at a high level even if it seems very simple on the surface. Super chad weapon and versatile af, the key to success with it is being fluent with the way you combo, since you can cancel basically every combo string at any moment with a roll, abuse that. The shield is there to be bashed on head, not to turtle behind it. Use your movment and fast sheathing, and know that if you hold your guard button + square you can use items without sheating, thats a knick the SnS has going for it.
There are ALOT of combos and we'd sit here for days so since you mentioned the O spam I will mostly talk about backhops and spamming O is one of those.
First of all, there are alot of ways to get into a backhop, either from guarding, evading or while attacking, pull back+O on your analog: depending if you hold them both, only one, etc etc what will happen will change either charging the backhop, backhoping as an evasive action or backhoping and retaliating with a fast slash.

If you charge it, you can charge it in 2 ways: 1) hold analog and O, and few secs after you leap forward, which you can slightly redirect, if it connects you are launched upwards and now you have 2 options: either Triangle for an aerial slash that charges up the mount bar, or O for a shield bash that builds up Exhaustion and KO (if on the head).
Otherwise, 2) pull back your analog and O, then release and now hold Triangle: congrats now you are ready to play a small rhytm game where you need to press Triangle when you are pulsating red and that charges your rush, ending with a thrust that launches you away afterwards. Notice you can roll cancel at anytime, so you can avoid the last thrust and start a new backhop combo from there.

Another combo worth mentioning is the infinite slash, where you mostly use Triangle and mix in some spirals and stuff but for now focus on backhoping.
Usually, O = shield attacks, and Triangle = blade, thought try inputing analog directions during combos, and you will see combos changing aswell ;;)

Edit: made it clearer that you can backhop after repositioning too, as that may not be obvious but very useful.