I know this can happen if a script is too long,
have you tried going into preferences(Alt+S)
going the the script tab and enabling the following:
Auto Closing Brackets
Enable Autocomplete
Enable On-Type Autocomplete
Extensions in VSC can’t do that type of autocompletion for you, it’s a feature specific to Roblox Studio. You may want to make a PR on the Roblox LSP GitHub or ask in their Discord if they’ve got one.