Your idea rerouting tile flows into a separate sump, and away from the septic system is very sound. The dilution of your septic system certainly decreases efficiency and could decrease the life of your leach field. If there is a way to separate the perimeter drains on the footing from the floor drain that goes to the septic system, it should definitely be done.
You are probably in a good position to judge the size of sump pit that is best for your situation. Either will work fine, but the larger pit will cause the pump to cycle less frequently. So if you have relatively large inflows to your sump system, on the order of 1 gallon per minute or more, the larger sump pit is definitely the way to go. A larger pit means more digging, so its less work to install the smaller one.
It sounds like your leach field is functioning well except for seasonal surfacing of water due to high inflow and soil saturation. I don't think you are at risk for the leach field for now. The biggest advantage of separating out the tile flow is that incompletely digested materials won't be flushed from the septic tank to the leach field. That is where you can have problems. This is an ambitious project, but it will preserve your system from expensive repairs or replacement. Good for you to think of it.