Change default browser?

You cannot change the default browser. However, since you can install Chrome on iOS, and use it as “default” for certain apps that can be a workaround. Google allows you to set Chrome or Safari to be the app that opens links from other Google iOS apps (Photos, Gmail, Inbox, Google Calendar, etc.) Tap your avatar at the top let > Scroll down to reveal Google App Settings and set the preferences:

I have an update to this, which I think is even better workaround. If you have Google App installed, you can enable custom keyboard GBoard in system Settings. Settings > Google > Keyboards > toggle Gboard to On > toggle Allow Full Access. On submission it will have a widget for search in Chrome. They do have a notice of that, but just in case, everything you type with this keyboard can be accessed by the developer.