Race: For long term progression you need to be a Nord Tank ideally. For the newer trials you’ll receive a ton of DoT and Bleed damage, the only way to manage this is making sure you’re at the resistance cap and by being a Nord that will make it very easy without having to focus too much attention into buffing your resistances.
Mundus: The Atronach providing Magicka Recovery is essential generally. For Cloudrest 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: When using Harmony Jewellery you might need to use Sugar Skulls to buff your Max Stats so that the gains from the Harmony Jewellery aren’t wasted. If you want more recovery then Red Frothgar is a great food but massively lowers your Max Magicka and Stamina. If you have Stamina skills that you need to cast often and you’re experienced and know when it’s safe to drop block then Smoked Bear could be an option.
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: As a Main Tank you’ll be the person in charge of almost every single fight in every trial. You’ll received the main brunt of all damage and so you’ll want to stack up your resistances as close to the cap as you can with Major & Minor Resolve, Passives and Champion Points. Using some Reinforced gear or Defending One Handed Weapon can also be beneficial.
Magicka Recovery: You need approximately 2000 Magicka Recovery as a minimum as a Main Tank to be able to sustain a 100% uptime on all of your skills, buffs and debuffs. Having more will mean you can utilise some skills more frequently such as multiple casting Igneous Shield for sustain/shield for example.
Stats and Attributes: For Trials Main Tanking you should aim for 40k Max Health which is a general comfortable number for most trials. Max Magicka and Stamina comes down to preference. You could opt for even more Health if you wanted to attempt spending less time blocking and giving a bit more safety. It’s likely as a Main Tank you’ll spend portions of time holding block meaning no Stamina Recovery so it’s good to have a decent supply of Max Stamina to allow for more prolonged block periods when necessary as a minimum around 20k but up to 30k can be useful if you block more. Max Magicka is the least important since you really stack Magicka Recovery so you have a constant resupply of that stat, but if you don’t carry enough Max Magicka you can have quick burnout phases. If you have only 10k Magicka you’ll potentially run out after 3-4 skill casts so aim for 15-18k minimum.