Keep up the good work Mento,
Of the three, Mag is a more difficult starter warframes. Especially at the point where you are in the game Mag is a bit under powered compared to Excalibur. Don't get me wrong, I am bit of a Mag (Prime) "main" (it's my most used Warframe). But to save you some potential struggle I suggest maybe coming back to Mag until later when you have your mod collection more flesh out.
@blacklagoon brings up a good point about trying to get Valkyr. A tip for fighting Alad V. Use Mag's magnetize on Alad V or Zanuka whichever you decide to deal with first The bubble will give you auto aim (zanuka likes to jump around). Shooting in the bubble will usually crit as well.
Valkyr is tanky like Rhino, but also fast frame.
As you alluded to Mag's 2 (Magnetize) is great at taking down bosses. Magnetize is secretly Mag's best skill, any projectile based shots is almost guaranteed to count as a headshot on any enemy trapped within the bubble. When the bubble explodes it does AOE damage to surrounding enemies. This damage scales with the amount of damage the bubble absorbed. The augment allows you to control the bubble, exploding it whenever you like.
It adds a CC component to shield polarize. In addition to Shield Polarize dealing damage to surround enemies based on the amount of shields the enemy has. (Which by the way scale damage strength)
Any enemy not killed by Shield Polarize will be disarmed for a short duration allow you catch your breath and pick them off while they fidgeting with their weapon.
Don't worry about using your max frames to progress. When trying to progress through the starmap you want to have one good weapon as a decent weapon, along with the weapon you are leveling as a good general rule.
Been talking to new players coming back to Warframe in Giant Bomb Warframe Discord. They also are finding that the Chroma fight is exceptionally difficult.
If you struggling with damage multishot mods help deal more damage as well. Combining elemental damage: ie. Corrsoive (elec+heat) or Viral Damage (cold + toxin) depends on what you have. Are you using your warframe abilities often? Do you have a mod like streamline or flow improve energy. management Make sure your leveling up Vitality and Redirection for increase heath and shields on your frame.
If I remember correctly, you don't need to kill Chorma, In the final confrontation with Chroma once you succesfuly defeated the corpus and defended the artifact, Chroma shows up and in order to defeat him, Chroma needs to be scanned 5 times with synthesis scanner.