The "intent" is indeed that you use your Job when playing in a group and your base Class (with its expanded cross-class options) to play solo, but few people ever bother. In general, once you hit 30, it's just advised to get your job and stay in it forever.

As suggested, Swiftcast is an absolute suggestion - being able to swiftcast resummon your pet or revive an ally in the heat of battle is incredibly invalueable. You don't "need" it for a long time, but give Thaumaturge some time when you have it to spare. As for Archer, I doubt you need Quelling Strikes or Hawk's Eye (let's be real, you shouldn't have problems with accidentally stealing aggro as a Summoner, and Hawk's Eye shouldn't really impact much ever), but you will want to get the couple levels required to get Raging Strikes, because it's a free 20% damage buff on a 3-minute cooldown. Pop that before you apply your spells on a boss and it's great.