What Calendar and To Do solution do you recommend?
I would like to have a central Calendar that I could sync everything to it, from my email calendars and my to do list, is there something like that selfhosted or not, that is FOSS?
In principle any DAV client should work. On Android, I use DAVx5 and Tasks.org to access my Nextcloud. On Thunderbird + Lightning, you can e.g. use SOGo connector or TbSync to use the address books, calendars and tasks.