You've received a lot of good info on this thread. Just wanted to throw in my experience with 100gal I setup, no filter with only a power head to move water around. And no offence to OVT, but it didn't smell.
I know you have experience with Walstad tanks, way more than I do.
Tank was an old saltwater tank, the kind with those way too small compartments in the back corner outfitted with way too little bio-balls and a drip tray that you had filter floss in. Bad for saltwater but great for fresh. When I made the switch to plante, I removed all the bio balls and just had the pump moving water around. It was enough to make the leaves sway a bit. Really wish I had pics of it but it was back in the early 2000's and my pics from that era did not survive the several moves I have made over the years.
I planted it with a bunch of different crypts as well as some different hygrophilias and a large group of Vals. The ones that were the most prolific was sunset hygro and the Vals. I also employed the use of duckweed as a nitrate sponge(hygro did well at that too).
Substrate was just regular aquarium gravel. No soil. Although it had been in a tank for awhile collecting detritus.
I stocked it with 100 cardinal tetras and a largish group of sterbai tetras(I think it was atleast 12, I'm remembering more but it's been a few years) as well as a smattering of other fish (gourami or 2, some rasboras). Funny part is that visitors always remarked on the red male betta I had in there. Never on the large school of tetras.
The key to this tank doing well was the growth of the plants. I didn't add the 100 cardinals until the plants, specifically the sunset hygro was growing well and I was trimming once every 2 weeks (no co2 in this tank). I didn't have any perceived die off with that addition (it is hard to get a good head count on 100 tetras). And the key to having good plant growth is to remove dissolved organics and add fertilizers that the plants need. For this tank it meant atleast every 2 week water change to remove accumulated mulm. And I used plant tabs (no water column ferts) roughly every 5-6 months. I could tell when it needed more as the growth slowed. Your tank will definitely be different. Even more so as you are adding co2.
I can remember letting the tank go to 1 month between water changes from time to time and that's when plant growth slowed and algae growth really ramped up. I'd do a couple big water changes, really concentrating on removing dirt from the top portion of the substrate and around plants and algae growth would subside and good plant growth would kick into gear.
If you take anything away from the above, it's that it's really important to remove dissolved organics from the system so that your plants can grow. That growth is the filter. In my opinion one of the best out there.
And I second the 1.0ph drop (guideline, not hard and fast rule). Do yourself a favor and spend the money on a ph meter that sits in the tank 24/7. They're not that expensive and really give you a better insight as to what your tank is doing at any given moment.