I haven’t really messed with melee Astarion but I feel like the game does not encourage the Sebille gameplay.

For this, the low level cap really is what hurts. If you don't mind being slightly underpowered for act I, going Shadow Monk/Assassin will get you Sebille-like gameplay. Shadow Monks get teleportation relatively early on, and their skills complement any class features/feats that increase sneak damage multipliers.

They're still over all outclassed since Larian's changes to the 5e ruleset and features really don't favor stealth for the most part, but you can make something close to a melee sneak build viable especially if you're not playing on tactician.