Unless it got fixed in the latest patch, you hitting a wall on Phobos is probably at least partially due to a long-standing balance issue. I forget the specifics, but some of the Grineer units on that planet hit way too hard. Last time I read a thread about the issue, I decided to check it out for myself by doing an excavation mission and, even with my maxed out, end-game viable gear, keeping the excavators alive was a nightmare.
That said, potatoing your favourite gear now is a good idea. For tougher Void missions, sorties and the like, you'll eventually also want to install multiple formas in your firearms. (Most melee weapons can be end-game viable with just a catalyst.)
I'd advice against installing a reactor in Mag, though. Her Shield Polarize ability makes her top tier versus Corpus, but she's not that great versus the other factions. Incoming rework might change that (maybe we'll get details in today's devstream), but, for now, you'd be better off switching to another frame (Rhino is a good beginner's frame) to take you through the rest of the star chart.
Not familiar with that PS4 bundle you mentioned. What melee weapon did you get with it? You should definitely consider upgrading from the Dual Ether. Also, if you got platinum with that bundle, maybe buying Frost Prime through trade chat instead of Prime Access is an option. Probably cheaper to get him that way.
It's the Grineer Elite Arid Lancers. They hit 3-4 times as hard as level equivalent units on Ceres. They are literally broken and have been for some time. They kill faster than the arid heavy gunners even when their gorgon rifles are fully spun up. The reason why it took so long to address is partly due to the community itself. People on the official forums are way too quick to say "git gud" instead asking more questions about the claims of imbalance. People have sporadically brought up that Phobos Grineer dropped low armor frames in an instant as well as making frost practically mandatory to allow extractors to last more than a second. naturally, some vocal People assumed it was all just bad play until someone actually captured footage, proving it.