First, please wait some days before starting the install, NC25 is under testing and should be released soon.
Also, OnlyOffice is no longer supported by Murena. You still can add some office solutions on your own, but there is no way that will fit in a CX11
2GB RAM may be not enough, please read requirements here: https://gitlab.e.foundation/e/infra/ecloud-selfhosting#requirements.
Also, as the requirements states, I strongly advise for a minimum of 2 CPU.
CX21 is the best starter choice at Hetzner.
As for tuning, I’d start with official documentation: https://docs.nextcloud.com/server/25/admin_manual/installation/server_tuning.html.
Please be aware that tuning to fit a whole selfhosted in a small box, always comes with a cost on performance and user experience.
As I remember, advised method to use external network storage, by both Nextcloud and Hetzner, may be to use Nextcloud External Storage instead of system mounts.
Also, you may have to check if system mounts will work with Docker volumes, you may have to use a specific driver.
A swap file may be of use, I’d start with a small 1GB and watch the use over a couple of weeks.
The problem here is not only the size, but also how it’s used: as it is “slow memory”, you may be facing problems if there is a lot of swap-in and swap-out moves. In this case you may consider tuning your usage, or adding some real RAM.
There are plenty of guides around about swap tuning.
For your information, this is a test server (no real users) on a CX21:
You may quickly find that the main memory eager is mailserver. The culprit is clamav, so you may want to build your own image without it
NB : this is a test server, no real traffic.