Thanks for the reply, but I think we're talking about two different things.
The more I research this, the more I think we're attempting an advanced strategy that simply isn't going to work on TOS, at least not unless I can code some kind of external management system to float on top of TOS. While the volume of the individual legs is extremely high, the volume of contracts using my synthetic spread is, well, just me.
I'm reaching out to some of the local CBOE guys here in Chicago to ask questions. I appreciate everyone's time and contribution to my question.
There is no NBBO in the SPX. It is a single listed product. When you send a spread order through TOS to the SPX, it will be sent to the exchanges COB (Complex order book). Larger orders in the SPX (non weeklies) may be sent to a brokers par station in the pit first. So for your smaller orders, it doesn't really matter your broker, the fills will all be the same here with all brokers as they will all be sent to the COB.
MM's will look at the order as a spread and determine whether there is enough edge to trade it. You are generally far better off sending an order as a spread rather then legging. I have found the spread markets very tight here. Doesn't matter if there is little or no volume in a leg.
Even if the MM's electronic systems don't think there is enough edge in the spread, it will still "auto trade" against the quoted market. So if the spread can be filled by buying the offer and selling the bid, it will do so. This is different from equity options that are multiply listed.
So the only reason you aren't being filled is you are not giving up enough edge. If the quoted bid and offer will fill your spread, it will fill at that price or almost always better.