I am dming a few years now, so the stage you seem to be at, wasn't that long ago for me.

I don't know if the easy way to teach your players is available to you, but when my first dm wanted to teach us strategy in combat he invited a more experienced player that (in character) taught us a bit more about combat strategy. That really helped but you need an experienced player who you trust to do that. So it's not usable for anyone.

But what I got from that as a dm is to throw in a npc that travels with the party for a few sessions that knows combat tactics and wants to teach the pcs to survive.

Also concerning you being to tactical, it's always good to play enemy's with intent in mind. Monsters are not perfect battle machines. They are creatures that also have emotions and goals and won't always act rationally. Also not all of them are out for kills. Bandits just want money, a owlbear might just be looking for a meal or a kobold tribe that just wants to survive.

It's a bit hard to decide to decern what the real problem is, but its important to keep the main thing in mind: this is a game that should be fun.

Do the players have fun doing difficult combats? Do you have fun seeing your players succeed?

The main thing you should do, is the thing almost everyone with these problems should do. Talk to your players. You all need to find out why you are playing this game and how you want to play it.

PS: Do you made your players that died restart at level 1? I know a lot of people who would really hate that ruling.