Check your DHCP Server and disable DHCPv6. The raspi shouldn't get a ipv6 then and you won't have this problem in the future with other devices.edit: and restart the pi after disabling DHCPv6
Go to android settings > apps > special app permissions (the last option) > alarms and remindersThe app causing the issue should be there, among others
Some apps that do some kind of scheduling display the alarm clock icon.Do you have any app set to do something at a given time? Maybe an automated backup or some kind of automation?
For your usecase a pi should be sufficient. You can go with a pi 5 8GB and a docker install, so you can host more stuff later. I would recommend an m.2 head with an SSD instead of an SD card though. Much faster and more reliable.
Check your DHCP Server and disable DHCPv6. The raspi shouldn't get a ipv6 then and you won't have this problem in the future with other devices.
edit: and restart the pi after disabling DHCPv6