Finally found the time/patience to hang out in a 30+ minute beta dungeon queue to try out healing on an evoker, and it was great. I doubt I was great it being the first time I'd healed on that class and the first time I was in any Dragonflight dungeon, but we made it and by the end I had a much better feeling for how the class worked. It definitely wasn't everyone's first dungeon, or experience with an evoker healing; I could tell as soon as I cast my first dream breath and saw all the DPS run into it.
I suspect the long queues and lack of enthusiasm from tanks, and non-evoker DPS, might be directly related by everyone wanting to try out an evoker. If it is more likely that not going to be one of the healers first attempts at a dungeon I could see how that could be off-putting. Shame there isn't an evoker tank spec as that would almost certainly decrease queue times, and give me one more reason to main an evoker.
That said I'm definitely going to main an evoker in Dragonflight, confirming I enjoyed the healing spec and running a couple dungeons was all I needed to finally make a decision. I'm still slightly torn on my second main/primary alt, but leaning toward my demon hunter. There is a nice symmetry of having "ranged" DPS/healing on one toon and melee DPS/tanking on another, plus I really enjoy demon hunter DPS, much moreso than druid DPS. I do prefer druid tanking to demon hunter tanking, but enjoy the overall demon hunter experience more; I have tanked a bunch on my demon hunter and the only bad thing I have to say about it is that I don't feel I have as many oh shit buttons as on my druid. I'll miss travel form and stealth, but with limited playtime some sacrifices for the greater fun are necessary.
Back to the preservation evoker, so my first dungeon (something with gnolls, freeing tortollian and corruption) I managed to completely forget about spiritbloom despite it being exactly in the right spot/hotkey. I had tried all the healing spells out in town about a week earlier, but it's hard to get a feel for healing without actually healing a real group. I feel that with DPS and tanking you get a better feel for how to use your abilities just doing world/solo content. In hindsight I could have tried to find out where to queue for one of those solo instance things for medaling in a certain role, I remember those from Pandaria/Warlords, but I'm not sure if they are still relevant. Lack of preparation aside it went well, no wipes, I never died nor did the tank, but I did lose a few DPS here and there. Most if not all those deaths were at least partially my fault, but people just released and ran back without comment. I didn't end up using a rez until the last trash packs before the last boss. I didn't know it was the last boss until we'd killed it and stopped as I was focused on other things.
After my first dungeon I tweaked my healing bars a little based on what I'd learned and requeued expecting to heal again, but I was assigned DPS instead. I suppose it makes sense there would be a lot of healer given the new class to try out. DPS I was used to by this point and it gave me the opportunity to look around a bit more and try out some of my utility abilities. It felt really awesome to scoop that lumbering bear up from the back and plop them in the front with the other Dracthyr running between trash mobs. I also enjoyed helping out the evoker healer by using my own healing abilities when things got hairy. Mostly it just felt incredibly liberating to not have to constantly hover away from and/or knock back mobs while killing them. I could just hang out and concentrate on maximizing damage and get a feel for an actual rotation rather than constant improv. I didn't have any add-ons/meters installed, but it felt like I was doing well. The dungeon was some sort of blue dragonflight tower with lots of teleporters.
I tried my luck one more time after that and did end up getting into another dungeon as a healer, but the tank went afk right before the first boss and the group dissolved after we couldn't get another tank for more than a half hour. The only reason I stuck around that long was that I was playing with transmog.
So now I just have to wait until evokers become available in pre-patch so I can level up to 60 and get everything set up for launch. That will mostly be leveling up engineering in a couple previous expansions to get all the important gizmos like a loot-a-rang, MOLL-E and a Jeeves. Probably an invisibility tinker to sneak past the odd mob or two and maybe a Reaves as well. I'll also probably want to gather up some gear from the pre-launch event to catch up my evoker and get some transmog appearances on alts.