A rooted android phone with Lineage OS flashed on it will be the best for security. You can do this with the OnePlus line of phones, they're great.
Regarding email, there is no secure email provider, just the lesser of evils. Anything but Google. Fastmail, Protonmail, etc.
Sprint and Verizon and T-Mobile and AT&T are all compromised, so there is no escape if you must use the phone regularly. Choose the one with the best reception.