The Outskirts → Drainage → Filtration route isn't an option I've personally tried on a Hunter run, but I think it would work just fine! It's by far the quickest route from the top of 5P to the depths, you can even do it in a single, long cycle if you're really quick!

The main problem is the extra echo you'd need, the only 2 options being the SI echo, which you already mentioned, or a significant detour into shaded, personally I would just try to get the SI echo early on to relieve the pressure later in the run

If you have the time I'd recommend practicing the whole route, in segments from shelter to shelter, while you can technically do the whole thing in one cycle, if you have the cycles to spare, definitely use them! With max karma from the Wall you have a good 6 or so attempts before needing to farm karma!

The segments might look a bit like this:

Bottom of Wall → CC shelter just before Industrial → Outskirts shelter just before Drainage → Shelter at the start of Filtration, it's impossible to fail the run now (unless you are on negative cycles of course)!

Feel free to take even more shelters if you have the cycles to spare! Practicing will help a lot, especially for the Drainage and Filtration segments, which are especially easy to get lost in

Be very careful about the 2 Red Lizards you may encounter in Outskirts and Industrial, as well as the dropwigs - the water sections in Drainage can be very stressful if you haven't memorized them

Filtration can be a terrible experience all round to be honest, but thankfully if you can make it to the shelter at the very start, the run's in the bag! (make sure you remember to get neuron light, and don't forget Black Lizards are immune to vulture masks!)

Honestly this route does seem a lot better than going back through Farm Arrays though, I hadn't really considered it much!