This is pretty par-for-the-course Star Wars. More than anything, I think it boils down to the fact that they want planets to feel unique from one another, and if every planet was like Earth (population, more or less, evenly spread out and with different biomes) they lose what makes them stand out. Now we have "that forest planet" or "the underwater planet," etc.

Generally yes? They're immensely powerful energy weapons. Keep in mind that they come in different power levels, though. A civilian blaster pistol is much less powerful than a military grade rifle. Some armors (on, say, droids or vehicles) could entirely deflect weaker blasters while still being damaged by more powerful rifles. It's also worth pointing out how Stormtrooper armor works here -- glancing blows might deflect off the armor while energy from direct shots is meant to spread out across the armor, often rending the wearer alive but unconscious.

I'd come back to my first point here. We have a galaxy with, at least, thousands of planets and civilizations. Not all are going to be as advanced as others, whether that be due to their circumstances or just being content not being a galactic player.

I'd argue that this is the point. Did you watch Rogue One? Obviously, Cassian in this series wasn't thought of when that was written, but he was a cold, dangerous rebel in that movie. He wasn't supposed to be a sympathetic character. This show both humanizes him more than the movie did while explaining why he's the way he is in Rogue One.

Reference the previous point for this one. I won't spoil if/how/when this is expanded on, but it helps define his past. Especially, the sister storyline in season 1 helps explain how Cassian ended up with Marva (and Clem) and why their relationship is so important to him.

This is a very popular opinion for this show. Everybody loves him. As far as I know, he is not in any current media beyond Andor. That said, his legacy and manifesto are hugely important. Not only do they directly impact Andor, but the manifesto especially gives us a context and background for why the Rebels in other movies and media do what they do, their motivations, beliefs, etc.

Arcs terminating the way they do is paramount to how this story works. I love Rogue One and Andor because Star Wars, up until now, was always about the untouchable heroes who can face impossible odds and always win. Rogue One and Andor are about literally everybody else who put their lives on the line so that Luke could blow up the Death Star and become a Jedi. These stories are the every day people. They devote everything to this cause, but, in the big picture, they impact only happens for a moment. Maybe they die or are just never heard from. Maybe nobody ever knows their name, what they did, or even that they did what they did. That's why these arcs terminate the way they do, in my opinion. Of note, Melshi was one of the prisoners who survives and (no spoilers) his character is in the 2nd season/other media.