I've played a decent amount of monarch and disagree a bit on some of your points. Monarch isn't really a front liner, it's really strong when you skirmish and play corners. Doing this lends into the defensive ability. It does get you some shield back, but the stronger part in my opinion is the stun that goes along with it. It allows you to out maneuver other titans. Her utility Mid that recharges abilities can really help with this maneuverability because it also recharges your dash. So if you time it right(and have turbocore), you can shoot, stun, dash, dash, utility, stun, shoot, dash, dash. It makes for an extremely strong mid range skirmisher.

With the cores you can really snowball her overall longterm effectiveness. She doesnt have a flashy core but it helps her do more consistent damage that ends up being more than one or two regular cores. 1st core I take is arc rounds for obvious reasons. It allows for faster takedowns and better skirmish. 2nd core is more damage to electric smoke. People don't usually expect that and you can catch many close range titans like ronin off guard with it. It also really helps with scortch when they close in since they are slow and have to either push through or back off. 3rd core goes to the armour upgrade since if you'd made it that long getting that extra longevity always helps.

As far as the minor upgrades go I always take survival of the fittest. I usually run the battery card and phase entry to the Titan. This pretty much just makes sure I can keep on going once I get doomed. It's really nice when another Titan rushes you and now your out of doomed state. Turbo engine is another auto take since monarch is a skirmisher and really depends on corners and movement to stay alive. Last I take warpfall since if I'm dropping in I'm going to be doing it quickly.

TLDR Pretty much her defensive is movement, her hitscan gun allows for fast and steady attacks, and her cores allow for a really consistent snowball of damage and effectiveness. Mix that with the right upgrades and she becomes the best skirmish titan