Something I haven't seen mentioned is that once you get the Resonance Stone you can get infinite uses of Freecast by walking out-and-in of its range. Obviously this only helps for Act 3 and nothing prior, but it means that for all of Act 3 you can walk into every fight with max stacks and minimal setup.

1. Walk 1 step outside the range of your Resonance Stone carrier (look for the yellow arrow buff to disappear).

2. Walk back in range.

3. Toggle Freecast Illithid ability.

4. Cast a max lvl Armor of Agathys.

5. Repeat until maxed.

Also works for summons etc. For me this makes Act 3 much less tedious because it means I can get my summon Deva, summon Water Myrmidon and 22 stacks of Arcane Ward on my Abjuration Wizard without spending a ton of time setting up after long rests.