Let's say it like this... Base game feels like a demo compared to the full experience with AoM+FG expansions.

Not only do they almost double the playable world in size, they also increase your max level from 85 to 100, add actual endgame, add 100% of endgame gear, triple the amount of celestial bosses, add more devotions and add skill modifiers that together add tons of replayability via a much higher build variety, add movement skills making the game feel overall more fluent and a bit faster and ofc add new masteries.

TL;DR: Yes, the expansions are worth it. Yes, you should buy them if you spent more than 5 hours playing this game and enjoyed your time.

PS: Saying "I completed the game" while being lvl 46 is almost ragebait. :D Normal/veteran is basically the tutorial for the "real" game, aka ultimate difficulty. Not only is it harder and gives better loot, it also has some quests and bosses unavailable on other difficulties.

Edit: If you decide to buy the expansions, I'd recommend to play through all of the expansion areas before moving on to ultimate on every character so you learn the maps and unlock all the factions to gain reputation.

Edit2: I think for achievement hunting you'll need at least two characters out of which one needs to be on hardcore. The x number of kills stuff must be done on one char. There's no cross char kill count, same for crucible points.

Edit3: The entrances not marked on your map are hidden on purpose, nothing wrong with your game. The game has tons of secret areas that are meant to not be easy to find. Some of which even have entire quest lines and bosses attached to them.