as far as classes... thats a hard thing to answer since that is personal taste.
if you were to press me for an answer... i have 4 arcanists, you can be either a healer or a dps... _for__me_ thats mostly what i like... however my character on Balmung is currently a white mage (healer, an evolution of the conjurer).
that said... you can't really just play one class. to be a fully flushed out post 30 character, many times you want abilities that come from other classes. eg: swift cast, something you get from thaumaturge, makes your next cast instant. group shield from conjurer. detaunt, something an archer can give, also a damage buff.
note: not all abilities are available to all classes. ask or research them as to what apply.
thus my character that is a summoner, is also an archer and a thaumaturge.
my white mage is also an archer, thaumaturge and arcanist.
this does not mean you need to level all to max level. may skills come at 15ish... high 30s being the archer detaunt. once you have the skill you want... you really don't need to go higher. here is Maobih on Balmung as of tonight:
also... it might not be obvious... but... this can lead to things getting out of hand. here is Foviel on Cactuar my first ffxiv character:
here is an example of adoption of classs skills from other classes:
the level numbers are in the event that you get scaled to a lower class, what level will be where it will be available as a skill.
all this said... i would recommend sticking with a class once you are happy with that class. don't go class happy till your at least 50. the reason being... you get a bonus leveling if you have another class that is higher. i have seen numbers like 230% bonus exp... so leveling can go quite fast if you have another class in the 50+ levels. get the class skills you feel you NEED to adopt into your primary and stop.
also, note that the advanced classes require that you have another class at a certain level to start them. see this for examples: https://www.reddit.com/r/ffxiv/comments/1l29jn/all_job_requirements/
once you hit 30... you WILL want to choose an advanced class.
hope this helps.