Hi Terry, first, the Starlink cable isn't compatible with Cat5 since the Starlink cable is POE (runs power over the ethernet cable). For that same reason, you can't splice Cat 5 or anything into the middle of it. Because of the proprietary connectors, you just have to use their cable. Some have been able to splice the SL connector onto the end of another cable but that is beyond my skill set. I'm not exactly sure of your set up. Are you going from pole--->utility room with power--->house?
If so:
1) I would use the 150 ft SL cable (order it ASAP) from the dish on the pole, down and over to the utility room and modem/router. 150 is the maximum length for the poe cable that they have and it can't be extended easily. Their cable is not buriable so either run it on poles or use conduit.
2) Run Cat 6e or 7 from the SL modem/router to another router in your house. You can use Cat 5 but will lose some speed. You can run Cat 6e for 330 ft with no problem and can get buriable cable. Mine runs 275 ft. I bought Cat 7 on Amazon and it works great but 6 is really all you need. You can turn off the WiFi in the SL modem/router and use the signal from the in house router if it is better positioned.
My house is about 450 ft from my dish and this is pretty much what I did. It works great as long as you have PLENTY of clear sky for the dish. Good luck.