It should have at least a web and an android app, and sync between devices
Bonus point if it鈥檚 opensource and it鈥檚 integrated inside a calendar
I鈥檝e tried Business Calendar but I couldn鈥檛 sync between devices without relying on Google calendar
CalDAV / CardDAV servers such as Nextcloud or Baikal usually offer Tasks along their calendar functionality. These servers then can easily be integrated into desktop clients like Thunderbird or Android apps like Tasks.org and others.
Yeah, what OPs asking for describes Nextcloud pretty well. There are good prices for managed hosting.
I use jtx Board with a CalDAV server. Not sure on where the jtx devs are based but at the very least it鈥檚 open source and based on open standards.
I鈥檝e been trying notesnook after Evernote turned to shit. But it鈥檚 not great.
I am using todoist it鈥檚 simple but powerful. Some features require a paid account.
It鈥檚 from the US. I鈥檝e been using it for years but am now actively looking for an alternative.