Best way to connect PVC to Galvanized?

This is for the suction line on my sprinkler pump. I have a sandpoint well that has a 1-1/2" galvanized tee on the end of the casing. Currently I have a 1-1/2" galvanized nipple coming from the tee to a PVC union then another galvanized nipple to the pump. I cannot seem to make the connection at the union air tight. I would like to run all PVC from the tee on the well casing to the pump. I get the feeling the leaks are occuring on the male threads going into the female PCV threads. Using a few wraps of pink PTFE tape makes it better but does not stop air getting in.

Am I just as likely to have leaks screwing in a SCH 80 PVC nipple into the tee on the well casing?

Keep the galvy tee. Ditch the union and the nipple. screw a pvc male adapter into the tee. Use tape or pipe dope. You don't need the union. I you need to take the pipe apart you can just cut it. If it was mine I'd ditch the pvc also and go with PE

I was thinking about something flexible like PE, the slip fittings won't allow any air to get sucked in? That was my only concern. I would much rather go with the PE it would sure simpilify things a whole lot. I am planning to install a bigger pump and add two more sand points. Using PE will make the plumbing a snap.

Yes it will and less chance of leaks because no couplings. Just make sure to use brass or stainless steel barb x male fittings and double clamp them.