Im building a destruction necromage (nord) and based on limited perks what is the most viable/effective destruction spells to cast on the island. After leaving the island/collecting new dlc gear and you had the chance to change those perks to another type of element would you? Has anyone specialized in frost and used it to beat lurkers/deathlords/legendary dragons/ other bosses

Of note i may use potion loops to get better magic regen along with fire resistance but not to be 2 powerful. Im not a fan of helmets/masks and eventually the only circlet u will be using will be the crown for unlimited raise deas so i will use that excuse to justify the potion loop

It sounds like you're asking whether or not there's an advantage to picking one of the destruction elements over the others, right? I don't think there's a specific characteristic of Solstheim and the baddies there to justify switching from, say frost, to fire. This has been debated before and it boils down to your personal style (I really like fire destruction over ice and shock) and how much you are willing to invest. Shock takes the most experience and uses the most magicka but, in the end, it'll destroy opponents at 85% of their total health. The others, fire and ice, have to drop an opponent to zero health before destroying them. Frost has the advantage of slowing opponents (I hate that when I'm trying to escape) but that doesn't appeal to me a whole lot. Fire does damage as the victim burns which gives your magicka a (slim) break to regenerate.

For my two cents, I suggest you go with fire (fastest and cheapest) 'cause it's the best looking effect of the three.

Thanks. I liked the idea of using frost for my mage but it appears that too many creatures have resistances to it. I think its fire and then any spare perks for shock

Fire is a pretty safe bet. Other than the occasional Fire Dragon, Flame Atronach, and Dunmer, most enemies aren't particularly resistant to it, and undead are actually weak to it. Combine that with it's DoT effect and it's cheap cost, and you have a solid element for most enemies. Just make sure to have a backup for the Dragons and Atronachs.

Frost is generally a bad idea. It's resisted by Frost Dragons, Frost Atronachs, Nords (which are quite common, since this is Skyrim), and Undead (also quite common). I will admit that the slow effect is nice, though.

Shock is generally a pretty safe bet, as the only thing that resists it is the Storm Atronach, which is pretty rare. In addition, Shock spells have no travel time (something that is unique to them), so they hit whatever you're aiming at the moment you release the control. However, this is countered by their higher Magicka cost, and I believe that they also have less range than the other spells, as well.