A very valuable use for extended spell as a metamagic in 5e is actually your ability to set-up for a difficult battle the night before. You can use spare spells slots and sorcery points (shuffled around flexibly as needed), to cast long term buffs with extension, so that they're last you through the next day, and your major battle, when you've slept and regained the slots you spent on them. Some primary candidates for this include:
Mage Armour
Aid (High level aid increasing the max hp of the party is super valuable)
Death ward
Darkvision
Foresight (!! If you've got your ninth spare at the end of the day, casting an extended foresight and then sleeping to get that ninth level spell back is amazing)
This is one of the most valuable ways of using Extended Spell, but it's unlikely that we'll actually be able to do this in BG3, sadly.
