well, I’m no expert, but I would recommend you check the related 3GPP docs (called “non access stratum, 23.122”)
With my little knowledge about this topic, I know that you just need to get the strongest RXLEV, and you should not restrict the selection (in other words, use the RXLEV ACCESS MIN at the lowest value -110dBm, and a MS TX PWR MAX CCH @33dBm, not more !!)
Also, try to place your PLMN as a “friendly” PLMN in the SIM cards of the roamers. To do that, AFAIK, you must ask the “roaming” operator to build SIM in this way. It’s a kind of roaming agreement I assume ? It’s definitely not something that will bring more revenues right away, but it can pay in the long term.
Also, you can advertise in the airports by telling the roamers to select the network manually.