I try to be tactical, but it seems impossible. I set up a solid bunker, mount my machine gun, and wait for the enemy waves, but less than a second after an enemy appears, I'm instantly dead.
Being tactical in a game like Battlefield is different from being tactical in a game like ARMA. Your ideas are mostly good, but it's a flawed execution.
Thinking about setting up a solid bunker has some merit to it. You are thinking about reducing of reducing the amount of angles you can be shot from and you are reducing the visibility. Mounting your machine gun has some merit to it as well. Machine guns do generally benefit from a mounted position.
Your primary mistake is waiting. The enemy isn't going to allow you to wait. Unless you only need to guard a single entry point, waiting just means that you're waiting to be surrounded. This is especially true in a game like Battlefield where destruction can be easily used to create new pathways.
Instead, you need to be pro-actively gathering information about where the enemy is. You can then plan a route to flank your enemy and properly distance yourself from them. That way you can catch them off guard and get a short window of time where you can mount your machine gun and actually start getting those multi-kills. It's essentially taking the steps necessary to give yourself the time you need.
It also feels like machine guns deal much less damage compared to assault rifles and carbines, which everyone seems to be using now.
This may very well be partially true. Assault rifles and carbines tend to have less damage per bullet and more damage drop off at range, but usually shoot more rounds per minute. So machine guns don't do as much damage up close, but will do more damage at range.
The game really needs to slow down a lot.
In terms of movement, Battlefield 6 isn't particularly quick. It's one of the slower Battlefield titles. The reason it feels like the pacing is really fast, is because the map design often forces short engagement distances, favoring weapons like Shotguns, SMGs and Carbines. Slowing down the movement sounds like a logical thing to do, until you realize that the movement isn't the reason why the game feels fast. Slowing down the movement only worsens the problem as you'd have more trouble getting out of danger and responding to unexpected situations.
When Battlefield Studios fixes the issues regarding the spread, it will somewhat lengthen the engagement distance and by doing so, it'll start favoring Assault Rifles and LMGs more. It'll become easier to engage enemies before they get the chance to surround you.
