Imho besser als Matrix, insbesondere für diesen Anwendungsfall: https://simplex.chat/invitation#/?v=1-4&smp=smp%3A%2F%2F6iIcWT_dF2zN_w5xzZEY7HI2Prbh3ldP07YTyDexPjE%3D%40smp10.simplex.im%2Fe8AX2DIXs-I8NyhXp78EWaqi5bM4M9DX%23%2F%3Fv%3D1-2%26dh%3DMCowBQYDK2VuAyEAubeSxurOHr0KTlcrVs3dle2V6ScAsm3QWEeuHFiq
A alternative to fairphone would be https://www.shift.eco/en/ They want to put killswitches into their new Phone which you can preorder for 535€ https://shop.shiftphones.com/shiftphone-8.html
It won’t be as secure as a Pixel with GrapheneOS. Grapheneos won’t support that Phone officially.