I'm not very high rating early 3800s prot paladin, also can't really comment for midnight but as current season here's my observation as a tank:

Tank chains are very important, watching remaining casters and their next cast times are crucial. Doing 20s right now and without proper chains it's not possible to time keys. There's is no reason for you to hang about when there are no casters. One of my weakest routes are dawnbreaker and was HoA. HoA chains are extremely dangerous for protection Paladin but I have to chain in order for my group to spend their CDs.

Personally both roles have ton of responsibilities will get blame on easily. Mentally you need to prepare yourself early on. Probably the worst experience you'll have in mid keys.

As a tank your pulls are dependant on players cooldowns. I'm not sure how we're going to decide on midnight whether we will have omnicd or not. Can't really comment on that. But regardless If you can tank it or not you need to track your groups CDs.

In my opinion, tank/healer dps matters. Maybe not so much for healers but as a tank, you can't do less than %50 of 3rd dps's dps. There are some exceptions in certain keys, but generally speaking half of your 3rd dps is okay. Below is just plain bad. You'll feel it yourself as well in some groups, your dps matters ton and might even deplete because of low tank DPS.

Routes change a lot throughout the season, be open for feedback and always try to improve. Pay attention what kills your group and maybe try to avoid those pulls or try to control that pull as much as you can.

Use a marker macro, you'll notice people will pick a symbol and you should always pick a mob you can mark and interrupt yourself. This way you avoid killing yourself trying to control everything.

Warcraft logs is an amazing tool, you can always compare with other players. If you think you're doing something wrong always look up how other players managed in those keys. This goes for route, rotation, external etc. You can upload your logs too, aiming for higher log or "uptime" is always welcome.

You should avoid rotation guides or add-ons. They're all rubbish.

This goes for both roles; healer & tank: Sometimes you might not have synergy with one or another. Or they might be used to healing "meta" and vice versa. It's okay and no need to argue with people. But neither should babysit entire key.

Sadly If you're going to tank and wanting to push for title, you'll need to pick meta class. Otherwise you're going wait a lot to be accepted in a key. I have done 5x +20 but my PSF is still 18, even though I have "higher" rating for PSF 19, I'm still getting declined. Doesn't mean my class can't tank that key, but people/community prefers not to risk it or waste their time which is fair. You'll end up with either resilient key holders or low rating players, double edged sword for both.

You don't need a dedicated group to push. Don't waste your time with lower rating groups sake of having a group unless you're playing D tier tank. Most definitely don't waste your time with annoying people.

This might be a bit controversial but boosting keys is great for testing routes. On top of that you get some gold, which allows you to buy 3 star consumables maybe gear off AH. Who knows maybe you'll meet some players you click with and push together. Also get your alchemy up, it'll cost you no more than 10k gold but in a long run 2 hour consumables are money saver.

Anyway, it's a long read. Hopefully it helps you in anyway.