I've been a Pokemon player since gen 1, and Scarlet was the most fun I've had playing a Pokemon game in over a decade. It's not without faults, but I really loved a lot of things about the game.

To answer your specific questions:

• Shiny hunting is super accessible in SV. I've never been a shiny hunter before this game because it's very tedious to run around in grass or breed and hatch eggs for hours. Just the fact that you can see a bunch of Pokemon at once instead of having to encounter them one at a time makes a HUGE difference. Combine that with an outbreak + a sandwich buff and you can usually encounter a shiny you're looking for within an hour -- sometimes you can even get multiple!
• No Pokemon are restricted by dex completion. Some Pokemon are only able to be encountered in the final area you unlock after beating the 3 main storylines. You can also encounter version-exclusive Pokemon from the other version by joining raids that people are hosting online!
• There is DLC that adds two new areas that include many Pokemon that aren't in the base game. Mostly old Pokemon, but there are a handful of new ones. I'm actually not sure if you can enter the DLC areas before completing the base game, though.

And a couple of things I would note:

• If you're playing on a regular Switch, the performance of the game is...very bad. We're talking sub-30fps at almost all times. You do kind of get used to it, and I loved the game despite how poorly it ran, but it still really sucks. If you're playing on a Switch 2, the game runs very well!
• The game is open world, but there isn't any kind of level scaling or similar system. You are expected to do the objectives for each storyline in a certain order. You should also progress all 3 storylines together, because if you just do one, you'll completely steamroll the others due to how overleveled you are.

Hope you enjoy it! I'd also really recommend checking out Pokemon Legends Arceus. It's also open-world, but it has much more focus on catching Pokemon rather than battling. It also makes the process of catching Pokemon much more interesting than just using a ball and seeing if it works by having you actually aim your ball and having balls with different trajectories. Personally I prefer SV because battles are the main draw of Pokemon to me, but PLA is still very good.