Being able to sync my work Outlook calendar with my private proton calendar is a must for me. I can set it up to sync and it pulls in the data, but it always becomes un-synced the day after and never gets any updates/changes to the calendar. This and a lack of todo list/task/notes functionality makes it unusable for me compared to google calendar at the moment.

Two way sync between apps, yes. Thunderbird integration with Proton Bridge would be a start.
CalDAV full support would go a long way in usability and adoption, as well as the ability to at least support credentialed iCal formats.
Additionally support for FREE/BUSY data information within calendar events is something else that is critically missing.

Frankly as it stands now, Proton Calendar is simply not enough except for casual use. Too many road-blocks and unsupported integrations stand in the way. If Proton Calendar interface and applications cannot support these things, then Proton Calendar MUST be supported READ/WRITE in many iterations and use-cases within OTHER apps that can and DO support all the necessary interoperability features between the different calendaring platforms. This at least would make scheduling within Proton Calendar as a back-end viable for work/life.
Again, as it stands now, this is just not possible except in the most multi-app and fragmented state.

Given the current political situation, I am very keen to move away from big US tech firms and onto the Proton platform. The lack of calendar integration (Google / Outlook/ Todoist) is currently a deal breaker for me however.

Please prioritize this update!

I've voted for a similar one earlier (someone linked it in the comments below). I need to be able to share my proton calendar with my husband's Google calendar.

Critical

I think this one is similar or identical to this one:

https://protonmail.uservoice.com/forums/932842-proton-calendar/suggestions/39383308-bridge-support-for-calendar-outlook-thunderbird

Real-time bridge between Proton Calendar and Google Calendar. I want personnal things added to my Proton Calendar automaticaly added to my work Google Calender with a "BUSY" or ""PERSONNAL" only label. That way, co-worker knows i'm not available, but don't know the exact reason.

2-way calendar sync please between proton bridge and Outlook.

This is the single biggest issue in Proton Calendar right now. I could avoid so many problems in the app if I could just use Fantastical or whatever.

I also use GSuite at work. I hate not being able to do everything in one client.

This is a breaking problem for calendar for me. I cannot use my calendar if I cannot coordinate and share it with family and coworkers not using proton.

While I understand that presents a privacy risk I feel like that should be a warning message I click "are you sure" on allowing me to accept the risk.

Since I can't share my calendar with non-proton users to collaborate, I'm stuck on alternative solutions.

I just want to be able to share my proton calendar with my wife's google calendar and get real time updates. We do not need to be able to edit each other's calendars, just the ability to see what is there continuously as we make updates. I'm also stuck using outlook for work, so would also be helpful to be able to see my calendar events in outlook (and vice versa) so that I can more easily plan my week. Unfortunately, not being able to do this, is extremely frustrating, and has me considering abandoning the Proton calendar.

Without the ability to share calendars with people outside of proton, the proton calends is essentially useless for me.

It would be nice to have this feature and determine what get sync down to fields or etc. This is feature that really needs to happen!

I think this will jeopardize the privacy part of proton and will be hard to implement securely since proton calendar is encrypted. If we allow this, even as an option and warnings, it won't be long since one of us will complain that something leaked and that proton promises were not fulfilled.

I understand integration with Outlook or Google would be bad for security, but my wife isn't going to swap over (or not any time soon) and we share our calendars through Outlook right now because she that's where all her work stuff is. The lack of 2-way syncing for Outlook calendars is going to keep me from really using Proton Calendar. The security bit could be handled by using an alias email and a specific calendar shared as opposed to our actual proton email and all our calendars?

not a good idea, more integration weakens security, the reason to move to proton is for more security than the big 3-4 provide!
Get a family plan and all profit from better security. I moved everything from MS365 and google to proton. Everything works smoothly and sharing in the family is a breeze. The biggest plus is, it reduces complexity! The suite beets anything MS has and does not lock you in on OS or devices (like apple does, btw working on an MBPM2).

I currently use Google Calendar together with my wife. I would like to switch completely to Proton, but this is currently not possible because Proton does not provide this function.

My wife doesn't use Proton and doesn't want to for the time being.

So, this is more of a question I guess. I wanted a similar feature so I could share my Proton with Google friends and family. I then realized that once my cal is in their Google via share, Google has my info again. My reason to use Proton is to keep big tech hooks out of my mail, plans to travel, and location. So, sending my proton cal or email for that matter gives the info right back to big tech.
Please tell me, do I have it all wrong?

This would allow me to truly leave the Google/Microsoft infastructure and not leave me on an island unable to communicate with others who are not willing to switch over. Business and Family calendars are a huge part of the infastructure that helps big data keep its claws in users. If you could create a way for interoperability between privacy conscious and others it would be a huge step forward.

I would be very happy, if i can integrate a Proton calendar in my business calendar (O365) - doesn't need to be 2-way-sync (only read is ok). I tried several way to bring this calendars together in a good way, but haven't succeed until now. Before i also missed the possibility to sync the calendar with Thunderbird, but this is solved for me with the desktop app.