Pelstata is almost always worth it unless you know exactly what you want, especially if you want to be useful in PvE.
If you plan on going doppel 2, you are going to ditch your shield in most scenarios. You can still pick one up and tank dungeons though if you did include Peltasta in your build. For PvP you’re always going 2h sword though. Highlander might be the better option for if you’re focused primarily on PvP. That’s really your call to make.
Peltasta is perfectly fine for spears. There’s no reason you can’t be a spear user with Peltasta circle 1 in your build. Many if not most dragoons choose to take Peltasta as their Rank 2 class.
You are able to swap weapon sets on the fly, so rarely is it going to hold you back. Personally, I prefer using 2 handed swords on my character, but I always keep a decent shield a mediocre sword along with me in case I end up tanking a dungeon (which is fairly often, admittedly).
You do get a lot more damage out of a 2 handed weapon compared to a one handed one. That makes sense. It would be awful if you didn’t. If you choose to pump strength, however, the difference can be negligible. It’s still there, but you’re not going to be worthless in terms of DPS if you’re wielding a good one handed sword/spear and a shield.
The main issue with choosing what weapon you want is that you have to use the correct weapon for certain skills. Once you know what skills are key to your build, you can check the weapon types and make sure they line up.
I hope some of this helps. I realize how confusing it can be to pick classes.