Returning player - a couple questions

Hey! Been playing a lot recently after quitting years ago and run across some things I'm unsure of. If anyone can answer that would be great!

Mega Pokemon - When I mega evolved a Pokemon, it appeared to take them to level 50 (I'm level 42). But with my Charizard, he only went to about 3400 cp. I checked the max CP for my IV and it should be about 5100. So is mega just not bringing them to level 50? Do I need to power them up to level 50 and then mega takes them higher?

Primal evolutions - I have many Kyogre and Groudons from back when they first released and now I see if I have primal energy they can do primal reversion. I keep seeing primal Kyogre/groudon at the top of their respective damage types, so I'm wondering if this works similar to mega energy? Meaning that when they bring back primal energy, will I be able to take the energy caught with the new Pokemon, and apply it to my older, powered up ones?

Dynamax/Gigantamax - is there ever a reason I would keep the normal version of a Pokemon over the Dynamax/Gigantamax? Obviously if the IV is better, I would keep that one, but wouldn't a Gigantamax Charizard have all the benefits of a regular Charizard? So shouldn't I be prioritizing these over the regular/mega versions?

Thanks for any info!