You cannot judge a job's complexity by it's APM. APM for BLM is very low but playing the job to it's fullest is a very difficult, complex task. Tanks aren't challenging because of their combo or rotational complexity, they're challenging because they're expected to do that stuff while also successfully tanking. The challenge inherent in tanking is about the macro-level engagement in a fight; boss positioning, planning for mechanics and cooldown usage. Aggro management used to be a thing you worried about too, but that was killed when ShB was released (and was on it's deathbed in SB).
A better option would be not to advocate for making tanks even more like a dps than they already are, but rather for a greater focus on tank-centric stuff in fights. Proper mob positioning is a huge one that is often neglected. Half the potential tank difficulty in MM is lost because Varis very politely jumps to the center before most of his mechanics, so say nothing of so many other fights.
I do not understand this constant push by a number of so-called tanks to turn tanks into glorified dps. This excuse of "well, we're all dps anyway" is bunk. Dps is not the end-all be-all for tanks and healers. Dps only becomes important once the tanks and healers have mastered the tanking and healing aspects of a fight. Sure, when those parts are mastered then you focus more on pushing dps, but no fight will ever be cleared successfully if all anyone does is throw out damage. No raid will make it past the first major mechanic in any fight if all the healers do is nuke, or all the tanks do is worry about GCD uptime.
As for cross-role dps comparison, I can't believe this dead horse has come back to life yet again, so I guess I'll have to kill it again. Ahem:
You cannot compare dps between different roles, nor can you compare dps between expansions. The calculations that existed for tracking damage in Stormblood are fundamentally different than they are in Shadowbringers, and once the switch to rDPS is factored in, the actual difference between tanks/dps in ShB and tanks/dps in SB is almost identical. The concept of "feeling" like a tank doesn't do enough damage is a completely delusional one. It's ridiculous to look at an arbitrary number and decide that it's not "enough." All tanks are clearly capable of beating all content in this game, so that means they work perfectly well. Adding X amount of extra dps to them will not change anything about how the tank plays, and serves nothing of value, either to the game as a whole or to job balance. It is literally a number on a third party site of questionable EULA compliance, and basing a sense of "fun" off of such a number is how bad balance decisions are made.
Put another way, you couldn't take 8 dps into any level 80 fight and expect to win. You'd be dead within a minute or so at best, and certainly no later than the 2nd major mechanic - with the possible exception of 8 RDM's, who might be able to zombie rez their way through a lot of a fight only to fail at the dps check because everyone's been cross-rezzing/healing. However, you could take 4 tanks and 4 healers into a fight and beat most end-game content. Probably not E7S or E8S, but with full ilvl 500 stuff I could see E5/6S going down. Ruby and MM would be a cinch. Hell, you could probably 8 tank Ruby and MM right now. And I don't even mean with PLD acting as a ghetto healer; 8 WAR's could keep Nascent Glint up full time on two separate targets. With ToB and Equi that's all the healing you could ever need in Ruby or MM.
But I digress. The point here is that tanks "deal damage" not just by pushing buttons but by enabling others to deal damage as effortlessly as possible. They are the ultimate providers of rDPS, as all dps is predicated upon a tank successfully executing their primary purpose. FFlogs has no metric to track how well a tank has enabled damage via proper boss positioning or mechanic preparation, but everyone who's spent even a little time at the end game knows exactly how much of a difference exists between a good tank and a bad tank. When the difference between an extra Kaeshi on phase 1 Shiva can be measured in fractions of a GCD, having a tank who's on point with positioning is a huge deal. How many thousands of dps have been lost by actual DPS because a tank is a bit off on moving a boss because they just had to get that last GCD in under Delirium/IR/NM/FoF?