1.) Main difference would be the combat along with several QoL changes to story progression. There is some additional content that wasn’t present in TSW but there is also a fair amount of missing content that you may or may not be familiar with. A fair amount of group oriented and PvP focused content did not get ported but story content is fairly intact.
2.) Population people can speculate on that all day since some people use the funcom client vs steam and all we really have is steam chart data. A good estimate is at least double what you see on steam charts. Or if I were to give it a ballpark number 200-300 players online on average.
3.) Endgame is good and it seems as far as updates go that is there focus but due to population you you won’t find much through random queing. It’s recommended that joining a cabal is the best route as most endgame content is organized within cabals. Another important note is that the gear grind is a bit more intense then it was in TSW. Depending on what your defenition of endgame is it could take several months to reach it unless you have a lot of time or deep pockets.
4.) General leveling is easier at least for story content. Beyond the story there is a tier system based on item power which now ranges from elite 1 - 17. It is a fairly long journey but I would say simplified compared to TSW.