I have 3 Anova units. 2 800 watts and one 900. I have used them all in containers up to a 33 quart Coleman stacked cooler and there is no real difference between the 800 and 900. Just make sure the container is covered to stop evaporation and evaporative cooling.
As for the app, it is the Joule that requires the app and that is why I don’t own one. The only reason to use the Anova app is it makes it much easier to set the built in timer, but 1) Sous Vide cooking rarely needs a timer, and 2) it is usually easier to use you phone’s timer or a kitchen timer. The only controls I use 95% plus are the on off touch button and the temp wheel. Plug in, turn on, set temp. No need to get any more complicated than that.
For salmon, it it has been commercially frozen to kill parasites, I cook it at 115 which results in a buttery soft flakey fish. Chicken breasts and turkey white meat I cook at 140. Thighs I usually grill but if I SV it is usually at 155.