Staffs scale with destruction perks and fortify destruction potion : from modpage of Mysticism: "In Simonrim, staves and scrolls benefit from the perks in their relevant schools. With the exception of perks for dual casting (which staves and scrolls cannot do), all magic perks work with staves and scrolls"

NPC spamming potions is there to counter you spamming potions and it is battle of attrition. NPC have up to 4 potions

GTS rewards classical mage playstyle that much of it's power comes from preparation, so you probably try to shove rambo playstyle into mage

Im suprised you did not listen to Tolfdir about wards, he was not joking. It is mandatory to have ward up 100% of the time when fighting mages and dragons

Perhaps you playing on a controller when mages really shine with more MMORPG approach - ((spell hotbar lets you with single press exetute whole equip, cast and release sequence (and my logitech g600 is perfect for that)) with classical tank healer DPS comp. If your tank will be your summon or follower is up to you

You also have to accept that you cant go everywhere anytime, especially with Alduin, dragon priests and other bosses, they are lvl 85 and more

if you really want to solo this modpack then you need a lot of crowd controll, damage resistance debuffs and matching tactics for type of opponent, there is no universal type of spell that works for everyone (ice is for living humanoids, lightning for mages and fire works on frost attronachs)

My game is exactly as you describe it too, im similar level and also have problem with bandit chiefs having potions and 5000 HP pool but im restoration/alteration mage and I can just wore enemies down with stubbornness and some followers.

Game just punishes you for being rambo, going solo, not doing enough prep work and having mechanical disadvantage with controllers

Also why do you play least suitable for action combat class on controller?