What I've done in the past is buy a good amount (150-200) alchemy master writs, level a character to about 12, level alchemy, and use Writ Worth addon (sorry console players) to auto-make all poisons and potions needed. Drink Mythic Aetherial Ambrosia + double exp potion thing, and hand them all in in one sitting.
Literally auto create all alch writs, get both xp buffs, and spam open and hand in alch writs. A lot of I and E button pressing, but can level up a char super duper mega ultra fast, and get skills levelled that way too.
Other than that, Skyreach is probably way to go. Spellscar and other places are going to be packed, and more people - the less exp
Difficulty scaling is desperately needed. 9 years. 6 paid expansions. 25 DLCs. 41 game changing updates including A Realm Reborn-tier overhaul of the game including a permanent CP160 gear cap and ridiculous power creep thereafter. I'm sick and tired of hearing about Cadwell Silver&Gold as a "you think you do but you don't"-tier deflection to any criticism regarding the lack of overland difficulty in the game.
Normal maelstrom is a good solo place. Should get you to level 50 in 5 hours or so if you are quick during your runs(assuming you have all training gear + 150% xp buff)