I'm going to throw my hat into the ring here, since there are certain things that I feel aren't mentioned or are objectively wrong. While Charmander isn't a bad Pokémon, as is the status quo for most starters, but it has some glaring issues that make it worse than some of its competition. Here are a few of them,
Poor Early-game
Charmander/Charmeleon is weak to Brock's Rock-types and Misty's Water-types. This results in either one of two things. Most commonly, you're forced to slap other Pokémon onto your team to deal with them, being Mankey or Double Kick Nidoran, and Pikachu or Oddish/Bellsprout. Either that, or you do what you went with and invest all your EXP into Charmander. This latter choice is worse, as just as you mentioned, you still need another Pokémon for Misty even with Charmeleon overleveled, which requires you to train up another Pokémon.
Overuse on Resources
As mentioned prior, Charmander needs to have evolved by the first gym for it to defeat Brock. This is a joke when the other starters, Nidoran, and Mankey could do the same at lower levels. Furthermore, Charmeleon/Charizard likes the use of the Earthquake and Rock Slide TM for coverage, but the same Pokémon listed prior prefer to use it themselves, as they can do so more effectively.
Mediocre Physical Stats
Charizard's only other Special move beyond its STAB Flamethrower would be Dragon Claw, which is a comical option for a coverage move. Otherwise, Charizard's Fly, Dig, Earthquake, Brick Break, Rock Slide, etc all work off its much lower Attack stat of 84, meaning it can't use its Special Attack as effectively as it should.
Elite Four
By the lategame, Charizard generally becomes more situational with its matchups. Despite what coverage it may have, Charizard struggles against the likes of Lorelei, Lance, and Blue's Blastoise.
So, while Charmander is not a bad playthrough Pokémon at all, it is outclassed by other Pokémon as being the best of the best. If you're willing to put down your biases for the three starters, I would recommend Squirtle more often than not. While I won't go into my reasoning fully here, Squirtle and its evolutions have the most consistent matchups throughout the Kanto region, only really challenged by other good Water-types for its spot, such as Starmie and Vaporeon.