Email is insecure by default; you can encrypt messages if the recipient is on board, but that won't be the case for most people. You can use services like Proton Mail, Tuta Mail, Disroot, Autistici/Inventati, etc. (or self-host), and they're all better alternatives than Gmail, just don't expect secure communication
Definitely Proton Mail. That being said, avoid using emails for secure communications as the standard was never meant to be secure. Sure you can use PGP but it has no forward secrecy, meaning if your private key ever gets compromised, not just one email will be cracked but every single one you've ever sent. That's ignoring the fact that your recipient almost certainly has never even heard of PGP to begin with. It's not quantum secure either and thus living on borrowed time. TLDR use Signal for communication whenever possible.
Could you explain the collaborative work you do with Google Office? Does it have to use office file formats? Is it required to be web-based and accessible via a browser?
Could something like Obsidian.md work (with some live sync related plugin), where you write in markdown for various note-taking and documentation formats? Obsidian is proprietary, and I haven't used it myself, but I heard good things about it. It's also possible to configure vim and emacs with plugins to provide the same functionality as Obsidian.
I personally want to learn LaTeX for my office needs, and I believe there's ways to do live collaboration with various tools, but LaTeX is a rabbit hole and much more difficult to learn.
What feature set are you looking for and are you wanting something free or open to paid?
I use both Mailfence and Protonmail different experiences with each but no real complaints. For Mailfence I pay them since I have a domain and I am still degoogling so forward my gmail to an alias there
Well both have free options as well, but they have a bit lower limits in that case and a few less features. So I guess maybe sign up on both for a free account and see which you like more?
I've started using protonmail, it's good so far but the storage is quite limited if you don't pay. Thankfully I get very few emails so that is not a problem for me.
I want to get rid of ProtonMail eventually because they may be a honeypot.
Protonmail Behaves like a CIA/NSA “Honeypot”
Protonmail has an Onion domain that allows users to visit their site using the TOR browser. Protonmail even has an SSL cert for that onion address even though it’s completely unnecessary. When a user makes a new account with Protonmail on TOR they are re-directed from Protonmail’s “.onion” to “.com” address. This breaks your secure encrypted connection to their onion address, enabling your identification. There are absolutely no technical reasons for this feature. In fact, the only other websites that operate like this are suspected NSA/CIA Honeypots.
This is a huge security issue that was either created because Protonmail is managed by Particle physicists who do not understand computer security OR they have been forced to operate their website in a similar way as CIA/NSA honeypots. Both possibilities are serious concerns.
I'm planning to self-host my own email service, but that comes with its own problems, including major email services banning most IPs that are not other mainstream email providers for the sake of "spam protection".
To hopefully counter this and to have a separate email for payments required to be able to host my own email service, I have an email on macaw.me and monocles.de (about monocles).
I have been on the search for another larger and more developed email provider as a backup for instances when I can't use my other emails because they are blocked by some services or an entity I need to interact with. Here's a list of email providers I have managed to find for you:
cock.li (Vulgar, but has less vulgar domain name options, and they claim to be more transparent and honest, but I get some right-wing libertarian vibes from this service as I learned about it from Mental Outlaw (yeah, I know I watch some cringey tech channels, but I learn some helpful device from them, which is the only reason why I still watch them on occasion))
Fastmail (Proprietary, but they contribute to open source projects, and their JMAP standard project is pretty interesting)
Forward Email (Free/paid and open source, including the backend, and quantum-safe encrypted)
Maildrop (Free and open source throwaway email service)
Migadu (Proprietary, but built with open source software and has some open source repositories (GitHub), but forced free tier users to switch to a paid tier within one month during the pandemic)
MXroute (Proprietary, but built with open source software and has some open source repositories (GitHub))
Runbox (Proprietary, but built with open source software and has some open source repositories (GitHub))
systemli.org (Apparently it's only available by invite link, and you have to be a leftist and know someone willing to give you an invite link)
Zoho Mail (Proprietary, but has some open source repositories (GitHub), have a good reputation, and also provides services such as an office suite, though their services are business oriented)
Keep in mind I haven't done much research on these email services to be able to recommend them. I am posting whatever I find, and hopefully someone may be able to make a comment or you will be able to research more about them to see if they fit your needs.
I'm going to post this as it lists some email resources, testing tools, etc. even though the curated list is more intended for self-hosting email and email design: