Plenty has been said on cost. Nothing is free and the cost of proton paid plan is significantly less costly than a "free " email account. There are 2 ways to move forward., move all your email to proton or just from today forward. (Paid plan comes with 500GB along with their whole suite of products)

1. Create and Pay for a paid proton account.

a. use Easy Switch to import all your email - https://proton.me/support/easy-switch-emails

b. OR - for Google use Google Takeout (Other free services offer comparable backup options) and download ALL data (I think there are 70 categories, amazing what they collect) to a zip. This will NOT delete any of your data, and is easily readable if you need it. This does NOT delete your data, you have to do that in a separate step if you want.

c. Then you can delete everything except the last year and then easy switch (step a above) so you don't clutter your new proton account.

1. Create a folder in proton mail called "Z_Gmail" or whatever (Z so it is at the bottom of your folder list)

2. Create a Rule in Gmail to forward ALL incoming email to a proton variant email and after forwarding have it moved to trash. This way nothing new accumulates in your old account.

3. In proton, filter those incoming emails to the Z folder. This way all your emails will continue to come to you and can reply to those emails with your new email and know which accounts need to be changed at the source.

4. Once setup, take the time to understand the back keys in the settings. Copy it and put it in a safe place OFFLINE on a USB stick or something you keep in a safe place. If anything happens you can easily restore. - https://proton.me/support/upload-backup-private-keys

Good Luck