I also have an iPhone as well as a GrapheneOS phone. They are different but I'm quite happy with those alternatives to the apps you mentioned:

Etar as a calendar frontend app. DAVx5 for syncing CalDAV and CardDAV (so calendar data, contacts and optionally also tasks). I manage tasks within my notes, so I do not really use a todo/reminder app at all but there are different options. tasks.org plugs into DAVx5, for example.

Depending on your server-side setup and your willingness to compromise on open source, are also other options. For example, if you work with an Exchange server, there is the "Nine - Email & Calendar" app which is a very powerful all-in-one-client similar to traditional Outlook on the desktop.

For photos, there are quite a few solid options, depending on what you want. I use the Fossify Gallery with only my Camera folder visible for day-to-day stuff and also have Aves Libre installed for a more advanced interface to my pictures and videos.