The issue I have is Proton Calendar customization and interface options are too limited for my productivity. I would rather use Proton Calendar but it takes me literally 2-3x the time do to anything due to either extra steps or additional work-arounds I need to accomplish due to the lack of features. This is the main reason why I would want read-write access from another app is so I can take advantage of other app features.

Maybe Proton can use a Proton-Bridge like feature for calendars like it does with mail?

+1

Honestly I would be happy with just a 1-way-sync (automatically adding new Google events to my Proton Calendar)

At the moment I manually use the import function (weekly), but that's certainly not ideal.

It is indeed very necessary! The other way around would be a great step forward: beign able to add/remove and see calendar events of 0365 and GMail from Proton would solve 80% of this issue without compromising anything.

I acknowledge the Proton privacy goal - especially for mail content. It is however critical for me that Proton provides an option to sync calendar and contacts to native device apps. It should be my choise to "expose" contact and calendar data to a non-encrypted application.

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).