Race: For long term progression a Nord will be useful for when you get onto trials such as Dreadsail Reef and Rockgrove, it’s not as vital up until that point but it will make a big difference by being a Nord for the newer trials mostly because of the increased resistances.
Mundus: The Atronach providing Magicka Recovery is essential generally. For Cloudrest and Asylum Sanctorium you can use The Steed. If you decided to go with a difference Race then you might need to use The Lady to buff resistances.
Food: As an Off Tank you have more room to play around with your food. For many of the ESO Trials you only Tank intermittently so using recovery foods so you can maintain your buffs and debuffs is usually the most essential thing.
Potions: Generally use Tri-Stat potions. You might need to use Minor Heroism potions for more experienced raid groups to get your Ultimate back faster. In specific situations there are other useful potions such as Immovability and Lingering Health/Speed potions but they are very niche.
Resistances: For an Off Tank resistances are more vital in some situations more than others. For newer trials where you have to Tank adds with incredibly high damage such as Rockgrove and Dreadsail Reef, you’d need 33k. When doing older trials like Maw of Lorkhaj and Craglorn trials, the incoming damage is so little especially for an Off Tank that you can focus more on sustain and damage output compared to defence.
Magicka Recovery: You need approximately 2500 Magicka Recovery as a minimum as a Off Tank to be able to sustain a 100% uptime on all of your skills, buffs and debuffs. Having more will help to maintain more spammable skills that offer additional benefit.
Stats and Attributes: For Trials Off Tanking you should have around 38k+ Max Health, but you can manage with less for older trials and more for newer trials. Stats are largely up to you.