I'm a bit unclear about the plans. But if I understand correct your trying to decide weather to go with a run to waste drip system, or recirculating drip system. And your concern with a run to waist system is how long 30 gallons of nutrient solution will last you. Well I would prefer a recirculating system myself. Large commercial operations do tend to prefer a run to waist system. But they have a very precise drip system that has very little water runoff. On a large scale I can see it being the best option. But it takes the right timer and settings, as well as dippers that have a specific flow rate (=$$$). Not to mention consent observation to make sure dippers don't become clogged, as well as the growing medium is being evenly moistened without to much excess runoff. How long it will last depends on such factors like temp and humidity, how many plants your feeding, as well as how big they are. But a huge factor is also how well you monitor/control the dripping of the nutrient solution.

As for leaving a little water at the bottom of the container of the drip system before it drains. It mostly depends on what type of growing medium you use, and how deep it is. That assumes the growing medium goes all the way to the bottom of the container (no air gap between the growing medium and planed water level). My advice is make it adjustable. You want to keep the roots moist, but not water logged. You'll also probably want to change the water level during different times of the year too, because the plants water needs will change with temp and humidity (and especially size). So my advice, is make it adjustable if you can. If not, run a test with the growing medium (no plants or nutrients), let it cycle through (non stop for a while) and see how wet the growing medium gets (down where the roots will be). Then let it drain and see how how wet/saturated the growing medium stays where the main root ball will be.

I don't know enough from your post to really make a conclusion about your comments on the DWC system. I don't know how big the plants are? How much water volume you have in the buckets? If your using a air pump? And/or what the problem is unless your just worried about dumping/wasting used nutrients? Again I don't know what type of water volume we're talking. But going through 5 gallons or so of nutrient solution a week for a medium sized tomato plant (even more for a larger plant) seems normal to me.

If you let water sit in your trays until it reaches an overflow level, be sure the space isn't accessible to bugs or you'll be breeding mosquitoes in no time.
unless you're sure the standing water is being replaced periodically, you might be setting up an environment for anaerobic bacteria. If you're not sure, it'd be best to not let any collect in the bottom.
normally when you run to waste, you use a much weaker nutrient solution.
I think you worry too much about rain water weakening your solution. Put marks on the outside of the container to indicate known water levels. (say at 1/2 gallon increments) that way if it rains, you can just shine a light inside your nutrient solution and see where the water level is at. then add nutrient based on the additional amount of water or, depending on time since last change, simply dump and replace. Of course an EC meter would come in handy here for returning your nutrient back to its original level, but it could be worked around as stated a few sentences ago. (this was said to avoid unnecessary EC meter arguments) You could also place a little cone around the plant that would shed most of the water from rain away from the stem of the plant and off the side of the bucket (or whatever your medium is in)


for you DWC tomatoes, hang a string down from above the plant and tie with a loose not to hold the plant up. I've experienced those same issues as you've stated. lesson learned; put a drain in the bottom and an extra hole in the top. the top hole allows both adding nutrient and looking at roots. make sure to provide a cap of some sort for the hole.
light brown roots aren't too much of a concern. almost black brown roots are dead ones. some nutrients will stain your roots and it could be algae growing on them. make sure to keep the inside of your rez as dark as possible.
using an air lift to drip nutrient onto your medium has pros and cons. it will help aerate you nutrient solution but how much depends on how far it 'falls' before hitting the surface of the water as well as how fast it /how much falls.be careful as this could yield a false sense that you're amply aerating the roots. remember the roots will still grow into the nutes below so we still need to ensure they get some air.
it will promote algae growth on the top of your medium.
a lower nutrient level in the rez will mean you'll need to ad nutrient or water more frequently.
it could also deplete the region of roots in the meduim of oxygen if you run it continuously.
As I type this I'm seeing more cons than pros though they can all be worked around, so then its an issue of what you want.

I have a recirculating drip system for my greenhouse. Works good. On about 18 plants (tomatoes/peppers), I have a 35gal tank, that works good, during real hot weather (like now) I will go through 20 gallons of water 3-4 days. On another system, 6 plants, I have a 16 gallons tank, it lasts about a week during hot weather. Both systems use timers to turn on/off small water pumps that I adjust according to weather (more water during hot times).

I try not to let water sit in my system as algae grows fast and causes the ph to go up. Algae is hard to clean out once started. Using about a 5" drop in 10', helps keep the water moving into the tanks.