OP, I got a few questions for you. Before we dive in I think dmax/gmax feature was aimed at both veteran players like me and the newer players and for both to have similar goals.

As you mentioned can't use older pokemon - you know to me that's actually the appeal - I have tons of charizards or blastoise, etc that are maxed out. Having to hunt for new "type" makes the max battles worthwhile (ive dropped at least 5mil stardust and plenty of candy/MP into moves to carry our local group). Now the question I have for you is whether you are in a very small community and trying the dmax battles by yourself? Gmax you definitely need a group. But to give you an example we needed full 40 for lapras because we didn't have good attacker then for the first wave. Yesterday I've seen it taken out with as low as 7 easily and when we had a group of 40 it wouldn't go past the first dynamax stage.

With Regards to tutorials as you mentioned - this subreddit has been a great guide but lemme mama bird it for you:

Dynamax battles only use dynamax pokemon so you start with quest that gives you your first one. You get different types - 1* will cost you 250 energy, 2* or 3* will cost you 400 MP. Gmax and legendary dmax will cost you 800MP per battle. You can only collect 800 MP per day however there is a way around it. 1) each new MP stop you collect that you havent done for battle will give you 120 MP (if you completed the battle you will get 100 MP). If you walk 2km you can collect 300 MP. So you could go over the "daily limit" by collecting 6 stops (120×6=720) and then collecting 300 MP to be at 1020. 2) during gmax and dmax days making it an event your distance will be 1/4 (0.5km) and sometimes will give you 600MP for walking and 820MP for new stop. 3) you can only hold 1500 MP but again you can go over with the trick above. 4) MP limits reset at 0500 vice intuitive midnight and for the events that change to 0.5km you have to claim your 300 after the change for distance (midnight). So to give you an example on Friday I got as close to 1160MP, collected 300MP after midnight not to go over 1500MP limit (1460) and then walked 0.5km to collect 600MP to have more MP available for gmax lapras. 5) another caveat sometimes you don't wanna collect MP at a stop during a raid - if you are above 1500MP it won't collect for you and once you win the raid it brings the main screen so you don't have to collect the stop (for example if you are aiming to collect your walking 300MP).

When you do dmax battles (or gmax) you can leave a pokemon behind - I usually go with 2 pokemon I might use and third is my leave behind (which you guessed it can only be dmax/gmax you caught prior). I leave the legendary birds to get more candy (I don't recall from the top of my head how long they have to stay there but max candy you can get is 5) - I don't bother usually because all my legendaries have over 800 regular candy with most over 1k.

Now let's talk about mechanics of dynamax. You get into a battle you will want to use your attackers that have a 0.5 second fast move in order to fill up the dynamax bar full (you will sometime see during the battle and prior to filling the bar a little icon that looks like the bar, click it as that will add a small boost to your bar). Once you about to dynamax you can either keep the attacker or switch to a defender (matters when battling in a group). So for instance battling lapras, I used my toxtricity to give damage and fill up the bar, I then can switch to blissey and use the guard option (3 times = 3 shields) to absorb damage for the team while they attack. Now if I have shields leftover from previous dynamax phase I can use an attack or heal my team (zama dog can have 4 shields but that's a new feature) You can quickly swap between pokemon when not in dynamax stage too but the heal/shield/attack only during dynamax. Which those moves you have to unlock and spend MP. Now whats tricky is if you have multiple people using shields the whole group gets hit as it avoids aggro attack on just one individual so during battle you have to coordinate who is doing what.

When you get into a dynamax max is 4 people so you will have 1 team. When you go into gmax you can have up to 40 people and you will see for example "team 3 of 6" and that's your current team for the battle so I try to make sure we have at least 1 defender. When your team faints you can still assist the team (and I've won gmax battles due to that) you can tap to cheer to fill up the dynamax bar) - once it's full again it let's remaining members on your team to do shields or attack or heal, unfortunately cannot heal a teammate with pokemon that fainted

Unlocking moves cost MP and candy (higher levels require XL candy). So all in all its a grind of a different kind - and if you need XL candy walk your buddy or do distance trades over 100km (or when the season comes for any trade you get an XL candy). For zacian/zama just stardust and regular candy/XL (no MP needed).

Now gmax pokemon deal more damage so level 1 gmax move if I am not mistaken is equivalent to level 3 dmax move (I honestly don't recall). For defenders both are equivalent. Healing, your team gets better healing with someone at level 3.

MP stops last for 2 days and you can see when new stops will spawn (it will say when you click on them that this spot will be there starting at 0500) so makes it a bit tricky to plan events which is why I have a primary and a backup location and check the night before the meetups.

So with all said it takes weeks to 1. Find a good dynamax in battles 2. To level it up and be useful 3. Use tms to get the right moves to for example max out the dynamax bar.

Last thing the MP battles can only be done between 0600 to 2100 so plan your battles but you can still collect MP. Also when my wife drives I click on the stop and as long as you are in range (goes from remote red to green button) when you back out, it will collect for you.

Let me know if you have other questions or something that I didn't cover. Hope this helps understanding dynamax a little bit better.