There will still be loading screens, probably vastly reduced loading times but I there still will be some depending on the game.
You can play it solo up to level 30 but the missions will get tougher, especially on higher difficulties. Challenging difficulty is solo-able but Legendary is incredibly difficult unless you are geared properly and know what you are doing. You won't be able to solo the incursions as they are designed for co-op play. Some people have solo'd certain incursions but again, you'd need to be geared properly and know what you are doing.
You can play it just fine on SP to be honest. The side missions are much more varied this time around and offer a good challenge. Enemies scale based on your level and whether you are solo or in a group. You can call in backup when clearing control points too and using cover is very important or you will die quite quickly, it is a cover-based shooter after all. It's all about using your skills correctly and watching the enemies, they will flank and rush if you stay in one spot.
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The Division has all the stuff that was mentioned as missing in the article, it was gradually added after release but it's there.
Game-pass is available on PC too along with the majority of Xbox games, other than that the Series X hasn't had much going for it this year other than upgrades of last-gen games.