I tried a fleet of about 7 phase freighters and 3 crew transports with all the relevant skills. It's not really viable. First, the reduction from phase has diminishing returns and is barely enough to cover the base 300 visibility. Second, ai fleets can still have powerful sensors. Third, if the system has sensor structure thing, thats up to 700 more detection. I've not tested this, but destroying or capturing it seems to invariably make the faction hostile and they always know it was you, despite being unable to figure it out when you nuke and raid their planets... Fourth, planets can have like 4 fleets oribiting them, spamming sensor burst. The "inactive" fleets that do nothing, like traders loading/unloading still detect you, which is really annoying. Finally, it seems raiding a planet attracts most of the patrols to it, so it can be helpful sometimes to do a light raid to draw them away. Asteroids and such aren't very helpful, very few planets have those nearby. You can draw in a small patrol and kill it to create a debris field to hide in. The fields don't seem to follow the same oribt though, if they move at all, so you have a limited amount of time to make use of it before the objective moves away.
I was excited for stealth fleet, but it's not as effective as I expected it to be, for how specialized and niche it is.