Thanks. I already looked at Flare. Even though primary device support has been implemented (https://gitlab.com/schmiddi-on-mobile/flare/-/issues/128), the last time I used it this feature was disabled by default, because the developer strongly advises user not to use it (https://gitlab.com/schmiddi-on-mobile/flare/-/merge_requests/94).
snd
It was pretty good. Unfortunately, I had to resell it, because I wasn't able to put my ISP router into bridge mode.
I am trying to reduce my dependency on phones/android since years and try to use Linux (NixOS) for everything and I would say I've been quite successfully. The only application that is holding me back from abandoning phones entirely at this point is Signal, because it requires a primary device and only the official mobile apps are currently suited for this task. Flare looks promising, and the PR has already been merged, but it is not enabled by default due to other issues. For online banking, my bank supports SmartTAN that requires a scanner, but this allows me to do everything via a browser.
I have to say as someone who uses NixOS I love systemd, because it makes a lot of things very easy. For example hardening services ( systemd-analyze security) or replacing cron (system timer).
I just bought the openwrt one a couple of minutes ago after using networkd+hostapd+nixos as my router for more than 2 years.

https://ibis.wiki/ ?