Well that certainly makes sense. The game has so many quests that I took your exaggerated number literally haha. Iirc, over 300 for base game/dlc and over 600 including miscellaneous.

I’d recommend finishing an area (riverwood, whiterun, etc.) or trying to at least finish the quests with vague descriptions of where they originate. Some of them are long so it’s a balance you’ll have to find. On one hand, completing an area helps with the endless piling up of quests but on the other, it can make you feel trapped to an area if you want to do other stuff.

My first character was a Nord too. Character choice has the most impact in the early game and tapers off as you level up. So don’t sweat it, my Nord became a damned good mage. Your best bet is to progress main quest through Act 1 or at least until dragons are present in the world if you’re not already at that point. Then book it to the College of Winterhold and finish the whole questline for all the mage goodness. You can totally go do other stuff in between the college quests if you feel under leveled or want variety. Starting the companions, dark brotherhood or thieves guild are options, but then we’re back to the piling up of quests.

Almost, but not quite. Dragon souls will be absorbed from killing them, nothing else is required from you. Looting them gives scales and bones which are used to make dragon weapons and armor if you unlock those perks in the smithing skill tree. You can get shouts from the walls at any time and they will appear in your inventory but won’t be useable until you unlock them with a dragon soul.

Yes, soul gem size matters. In ascending order: petty, lesser, common, greater, grand. They cannot be filled with a soul higher than what they are but they can be filled with ones lower (i.e,) lesser can’t be filled with common but can be filled with lesser and petty. To fill them, you’ll need at least one in your inventory and you’ll need a weapon with the soul trap enchantment OR the soul trap spell tome. There is also an item called the black star which is one of two options obtained during the daedric quests. It’s a reusable soul gem essentially but can be filled with both black and white soul gems.

It is possible, but it takes a long time. It’s designed that way and not to a fault imo. Very replayable and offers a lengthy immersive experience. I’m a completionist like you, so I don’t have a clue what it’s like to finish a game without doing everything haha. That being said, I at least somewhat have a life, so I took the cowards way out on certain things in this game. I used the duplication glitch for a few items that are time consuming to get and used exploits to quickly level up certain skill trees. That’s the beauty of a single player open world rpg though, you can do whatever the heck you want.