You are talking my dream system so I can probably help with links etc. The main difference is the goal at least starting out of having the tank as a preheater for the main water heater.
Here are the downsides on my list
1. Space taken. I am looking at mounting the preheat tank in the basement. Which also means I need to be able to get it down a stairs and around a corner. The house is designed to get a large chest freezer down but that is about all. A box tank that big eats major space in the basement.
2. Danger during a major tank failure. If I have 300+ gallons of water sitting in that and it suddenly ruptures in for example an earth quake suddenly I could have 6 inches of boiling hot water to wade thru to escape the house. If it is less than 140 degrees F probably survivable without horrible damage if I stepped fast but hotter still and I don't know if I can make it or not. If the leak is slower the concrete of the floor will eat enough heat to moderate it so the only real risk is a tank rupture. If doing a built in as the house was constructed would want a sub-basement within the basement capable of holding all the liquid in the tank if it ruptured with a bit of margin for error.
Now here is the video on the dream system. Start at one hour and 9 minutes in and watch the next 45 minutes.
Notice some things. By having 5000 gallons in this he has 2 months of heat storage. That would beat any run of gray days here I have had since watching close as the longest run was 17 dark gray days in a row with only about 6 hours of sun in that entire time. By having the ability to augment it with wood heat it would get by that. Notice a neat math fact. every time you double the dimension of tank you increase the surface to lose heat thru by the square but the heat storage capacity by the cube. Now I had been stuck on the word "tank" for years after seeing this because there was no way to retrofit that in my existing house.
Then I got to this video and now suddenly I am looking at parts I can carry down the stairs and assemble. Talking about a day and half of heat storage for a family of 4. Betting with just me I could easily double that at hot water levels and probably double that again if I let showers etc get down to 70 or 75 degrees
The above channel as well as many others gets me combined information
The goal is to combine this with a spiral solar collector laid down flat with the tube climbing 1" per 20 feet of pipe from the outer edge to the middle. Information suggests one 4 foot diameter spiral should almost produce enough heat to maintain the tank temperature steady most of the time. In good times it would heat it for use some. So thinking I need tubing to equal 2 4 foot diameter spirals for the base run. Now flat double pane window collectors top out at roughly 140 degrees occasionally pushing towards 150. So a 2nd collector doing evacuated tube system would eventually be added to heat the water on up giving potentially a 3 fold increase in heat storage capacity. If the systems are built to function as drain back collectors there goes the complexities of the heat exchangers and the need for antifreeze. While more careful design implementation is needed if all the tubing is PEX without any outdoor connectors it should hopefully be able to take freezing even if something goes wrong. Next question PEX for absorber? This article shows the PEX because of its thin wall and steady heat should perform nearly as good as copper. Had 2 other articles found showing very similar results but lost the bookmarks for them in a hard drive crash and haven't found them again yet so this information isn't a one off.
tubing heat conduction for solar collector use
Notice the materials can all mostly top out right at 200 degrees F. The EDPM is 200, the PEX is 200 and at my elevation maximum water temperature is 202 degree so worst case is within bounds is my thinking for a non pressurized system. Figuring to build the box out of 3 layers of half inch plywood and some steel support structure. 3 layers of insulation 2 inches each. Line the box with plastic sheet for leak prevention. Outer layer standard extruded polystyrene where it is cooler as it won't take the inner heat. Middle layer polyiso because it is good to 160 degree with another poly plastic layer for leak prevention. Inner layer rock wool so as to provide a wicking layer for any leakage(rock wool will no grow mold and provides thermal mass.) Since my box goal is just over 6 feet deep in water for 3 PSI at the bottom and rock wool is rated for just under 5 psi crush I am good that way too. The only pressurize part of this is cold water line going in and thru the preheater water tank and back to the cold inlet of a standard water heater begin with. An anti scald / tempering valve will need to be added to keep the outlet temperature safe. All of the rest of the water would be open to atmospheric pressure and thus no risk. Since the pressurized line is being heated by atmospheric hot water it could never get hot enough to be a problem.
The dream for backup for this is to build the barrel for the RMH slightly bigger than a normal barrel and spiral say 1 1/4" pipe up it in about 3 to 4 wraps with flat plate welded between to create a barrel. Most of the squish boom arguments seem to agree pipe that large is not dangerous in a convective flow system because it can't flash enough of the water to steam at one time to be a threat. Worst case it will sound like a percolator. If the DYI "barrel" is 3 feet in diameter and you did 4 wraps that over 36 feet of pipe to gather heat from the RMH. By adding the additional cooling it should actually rocket better slightly. To catch rust flake want magnets at both inlet and outlet in a side passage trap. To reduce deep blow back into the tank if there is a problem run the water inlet flow through a tesla valve. No moving parts and nothing to plug off.
The water tank gets 5 stratifiers. 1. Plate collector return, 2. evacuated tube collector return, 3. ground cooling/heating collector return 4. PV panel cooling return. 5 RMH return.
I have more links and thinking stored if wanted but this is already way too long.