Should I follow one of the Paths that they set or am I safe to just Customize?
Depends. Do you understand the basics of 3.5e? If yes, you can customize.
I have chars in two different servers. Wayfinder and Argonnessen
And other codexians play in other servers.
For population, here :
https://www.ddoaudit.com/servers
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About armored caster, I said that is amazing, but there are a huge problem. The loot which you receive still for regular casters hence you will gonna find very few armor. 90% of the "chest" gear that I find are robes. And when you finally get a medium armor, is rarely enchanted with stuff useful for casters. I"m lv 23 and still using my lv 18 armor as I could't find anything better. And yes, did all quests of keep on the borderlands 3 times.