We have fair number of people in guild who are online these days. Our raids have regularly had 25 people or more. However, there's only two raid tanks. So on non-raid nights, we've been scrounging for tanks to run Mythics and Mythic Keystones. I've volunteered to tank some of the lower keys.
It's quite a change from last expansion. In Battle for Azeroth, I barely touched Mythic Keystone dungeons, even to heal.
I think the scarcity of loot has made a lot more people willing to run regular Mythics for gear and then lower Keystones, rather than jumping right into the deep end. I've been running +2 to +5 keys, pretty much with guildmates only.
I've been using my healing armor--with healing Legendary!--and a couple of Ret/Prot trinkets, along with a Str sword. I've also just been using the simplest all-left-side Protection Paladin build and just trying to keep Shield of the Righteous up as much as possible.
I've healed for our raid tanks in Mythics, and they're pretty hardcore people who know skips and which packs you can pull together. Tanking was a little intimidating after watching them.
What I've found is that steady pulling of one pack at a time works pretty well. In Keystones especially, you need to kill a certain amount of non-boss enemies, so just clearing to them normally and avoiding fancy skips is good enough.
In my experience, wipes are what cause you to fail the timer, both from the time lost due to deaths and the run back and set up again. Pulling multiple packs, well, if you survive you gain some time, but if you wipe you lose a lot of time. One pack at a time, with minimal time in-between, seems good enough for now.
One tip is that some consumables, weapon oils and the lesser Desolate chest armor, are really cheap and last through death. So it's worthwhile just using those casually.
Tanking Mythic Keystones has been fun, and an interesting experience. I doubt I'd want to do it for randoms, but it's been enjoyable with guildmates.