I just came back from getting my Guardian FSD booster and the Ram Tah mission, and I'm really surprised at all the difficulties the OP encountered.
1) Drive around and find and raise all the pylons before trying to energize them. This not only gets you familiar with the layout but gets rid of most of the Sentinels in piecemeal fashion. They often arise in response to raising the pylons. Note that you have to raise the pylon(s) by the Data Terminal first, and the others after that.
2) PDTs are a hoot, a fireworks show while shooting the Sentinels down. No need to land close unless terrain requires it, as they have a generous range.
3) I don't recommend carpet-bombing the sites to energize turrets and down the Sentinels, as they yield important mats needed for most Guardian module fabrications. These are also rather ephemeral, fading out in just a few minutes, so it's better to use the SRV to scrag Sentinels and gather their guts (no cargo space since they are mats).
4) Gathering the artifacts (Relics, Urns, Orbs, etc.) is not really a paying proposition unless you have serious cargo space, as the most precious, the Relics and Ancient Keys, go for under 14k AGP, and even one in your cargo can be a pirate magnet (stoopid pirates!). I filled my hold on the Ram Tah mission only because they were there and I had to scan obelisks anyway (which requires artifacts), otherwise I would have let them lay.
5) Git gud with the SRV's vertical thrusters 'cause the Guardian Structures are not only twisty and turny but usually sport a few tank traps for the unwary.
6) Use both Structure remnants and terrain to help with the Sentinal fights, they really are pretty dumb and have a distinctive attack pattern you can suss out and use to your advantage, especially if you have a PDT umbrella.
7) Having done all this in a 'Conda, I can state that there was only one site where I could not land within easy driving distance, a mountaintop retreat I nicknamed the Eagle's Nest. Everyplace else just required patient scoping for the sweet spot(s).