@soudk Okay, I'll just go down the list and try to answer all of your questions, forgive me if I miss one.

For offline calendar, use Etar. If you need Calendar and contacts synced, use self-hosted NextCloud.

I don't use email, but I always recommend Tutanota to everyone, ProtonMail is extremely shady to me, I can clarify if you'd like.

Custom domains for email providers are worthless unless you plan on switching providers frequently or using the same email for business as personal.