Well, I can tell you about algorithms I knew when I became Expert. Euler phi, sieve variants, trie tree, disjoint set union, binary exponentiation, modulo inverse/ Fermet's little theorem, topological sorting, dijkstra, floyd warshal, bellman ford, hashing, DP (coin dp, knapsack, LCS etc), segment trees, BITs etc. I don't even remember using half of these ever in any contest, but these are more than enough. I learnt like CRT, Edmond carp which have no use in expert. Also I learnt some stuff for fun like Sqrt decomposition, Mo's algorithm, Heavy light decomposition, Centroid decomposition they also have almost no uses below 2000 rating problems. I also learnt KMP, LCA, binary lifting. Problems related to these topics are fairly common in 1900-2000s. KMP is found in 1700-1800s sometimes. These are not necessary. I learnt them because I enjoyed learning, also because by the time I was Expert I was already solving 2000R problems and I don't like worrying about missing out topics.

Yeah, I don't know either https://cp-algorithms.com/dynamic_programming/divide-and-conquer-dp.html

Maybe, but I may need to deal with them when practicing.

Thanks man