I don't shoot for TV Drama. I shoot concerts and studio sessions. I typically shoot about 500MB/hour per camera (so 1.5TB per hour). I have large RAID systems and LTO-6 backup, so not a problem. As for the time it takes for color grading--that's not time spent, but time invested. Getting a good look for the client is what it's all about!
Again, some TV shows do have that "default video" look. The Amazon.com and Netflix original drama series I enjoy most are beautifully shot, edited, and graded. The writing and acting is pretty good, too!
This is too broad a question to be properly answered. The "TV Drama" genre that I'm enjoying right now is really cinematic TV: The Crown, The Man in the High Castle, The Handmaids Tale, etc. Such TV dramas are very different than, say, Castle and Supernatural. These new Fujinon Zooms are certainly a breakthrough in terms of price/performance, so much so that you could probably afford a second 18-55 in case you needed two wide shots, or even a second 55-135 in case you needed two tele shots. Alternatively, you can get one of each of these zooms, and then pick up a prime or two to make your life easier when shooting on a gimbal (if you plan to do so).
On the 4K vs. 2K question, don't underestimate the value of reframing, especially if you plan to have one or both cameras on gimbals. In such cases you really want to frame wide and then crop to the stabilized image. But generally I find that it's much easier (and more stress-free) to frame wide and crop than it is to try to squeeze everything perfectly into the frame.
I don't have any experience with XAVC-I. I shoot everything RAW (on my REDs) and I love the colors and the quality that gives me.
Good for you! I watched about 10 minutes of the clip, and you've certainly hit the mark you were aiming for. You've proven commitment, consistency, and craft. Hopefully that will create confidence within your network to give you a larger budget to work with the next time around. Would be interesting to see a writeup on what you think you learned in the process, and what, if anything, you'd have done differently.
The one thing that's going to make the greatest improvement to your sound is having somebody actively monitoring the sound and making adjustments as needed. It is very tempting to believe you can "set and forget" the microphones and get good audio. Not really true. Find a partner who really wants to get good at sound, put them on your team, and you and they will quickly determine what equipment (and what techniques) yield the best results for your situation. But if you don't monitor and actively adapt, you are asking for trouble (or in need of audio overkill--not a budget-friendly option).
You have just discovered one of the reason that white clapper boards were once so popular on movie sets. Not only did they help with the synchronization of audio and video, but their white color could serve as a reference point. Some people still adhere to time-honored techniques. Others think it is better to save 10 seconds on set, even if it means spending hours more in post. It is up to you to decide what your time is worth and how to make the most of your film's budget.