Mostly I do direct deposit of my paycheck to Strike and have it set to autobuy 75% of that deposit immediately and the other 25% I trigger buys manually when it looks like there is a little dip.

My first purchase was on Coinbase and I have also used Kraken for a larger buy (wire transfer and buy)

Kraken worked well when I wanted to wire transfer a significant amount for a big buy to get started. I like Strike for dollar cost averaging. It is so easy. Everything can be automated, kind of like having money automatically taken out of your paycheck and put in a 401k. I do try to transfer my Bitcoin into my personal wallet within a week of the autobuy, so there is never much that isn't in my private custody.