Email was never designed to be secret. Think of it like sending a postcard, the message is written on the back for all who handle it to see. The major free email providers like Gmail and Outlook make no secret of them looking through your email to collect marketing data, map your contact networks, and then share it all with thousands of marketing companies and the US intelligence agencies. Even your on premises corporate email is exposed to viewing by tech companies once it goes out to third parties.

My recommendations for improved security and privacy

Set up a laptop computer as a dual boot system with your normal Windows 10 in on one partition and Linux Mint on another partition. Generate and store your private documents in the Linux Mint partition. Linux can read documents from your Windows partition (if it is not encrypted), but not the other way around. You might want to set up a small partition with FAT32 (or use an SD RAM card if your laptop has a port for it) that either Windows and Linux can read and write to.

Install Libre Office as part of your default Linux Mint install. (I have no confidence at all in the privacy and security of Microsoft 365.)

Do email, calendar etc. using the German based Tuta which provides the privacy assurance of the GDPR privacy regulations. See: https://tuta.com/ You can set up your own domain name on Tuta.

Install Password Safe https://www.pwsafe.org/

HOW TO USE IT

It is only secure while you are dealing with other people in the Tuta or you attached a pre-shared password.

Both ProtonMail and Tutanota claim to offer end-to-end no knowledge encryption, and maybe they do, but I want to add another layer of encryption on top of that.

Create your secret message in FreeOffice and encrypt it with a long, complex password. Save your documents and attach it to the email in ProtonMail sending your message as an attachment.

Save your passwords in Password Safe so you can copy and paste passwords. Passwords are a weak link as simple ones can be cracked by machines fairly easily.

You have to share these long and complex passwords with your colleagues. Put them in a text file, save them on to a USB flash drive, and hand deliver them to your colleague.

NEVER be tempted to starting surfing the Web or getting on to Facebook on your Linux partition. Web browsers are too vulnerable to malware to totally trust them.

Any highly component computer technician should be able to set this up and provide training on how to use it. You can also employ me to do it.

Just remember any email you send outside the ProtonMail (or Tutanota) environment loses the end-to-end encryption. The message that was encrypted with FreeOffice will still be encrypted but you have lost a layer of encryption and are starting to leave visible meta data.

This is inconvenient. You are right. What is important to you? Privacy or Secrecy? The choice is yours.