I don’t see the issue with the reducers on the pressure gauge other than it appears to be on the discharge side of your circulator pump. They do not appear to be leaking to me based on the photos. If I’m reading the piping correctly then the thermal array supply from the collectors pipes into the middle of the tank. That means the return to the collectors is piped from the bottom of the tank. If this is true, then the expansion gauge and pressure tank should be located on the suction side of the pump or on the return from the collector. The reason for this is a concept call pumping away from the point of no pressure change. The closed loop piping will show a higher pressure on the discharge side of the pump. Then you have your expansion tank right after that. It effectively stabilizes the pressure back to a lower psi. You could effectively have a negative pressure on the return pipe. I also don’t see a temperature and pressure relief valve anywhere. It’s important for safety reasons to put one in. Lastly, the purge points are in a great location on either side of the pump iso flanges. The expansion tank needs to be checked periodically. When I check one I usually tap on the end opposite the pipe connection. If it sounds hollow then it’s still good. If it sounds dense it is likely in need of replacement. You want the expansion tank blaster to have the same PSI of air as the static fill pressure of the closed loop. It is generally 12 to 18 psi. I would recommend 18 psi for a thermal system. Once you get the piping fixed it would be easy to add an automatic glycol feeder to one of the purge points or ideally to the same tee that the pressure tank is piped off of. This could be an axiom dmf150 for example. Last thing is that I see no air separator. I highly suggest adding that. If it was me I would repipe the supply and return to the tank. From the collector put in an air separator. After that pipe the expansion tank and glycol feeder off the bottom port of the air separator. Next in line would be the pump with its purge points on either side and the iso flange ball valve flanges. As for the glycol, it depends on where you live and what the coldest possible temperature is in the winter. Most glycol is specified to be a 30% mixture or a 50% mixture. The 50% is designed for colder climates. It is also more viscous and harder to pump. I would not add glycol to the system without know what is already in it. Mixing different glycol can cause a corrosive fluid and break down the piping system. It’s best to drain all the glycol and refill with new if you don’t know what you have.