Can I rely on visa runs until I can apply for an ED visa in Thailand?

Entering by plane is a bad idea.

You're probably fine entering by plane one more time. But after that use a border bounce agency to take you to the land border and get a fresh 30 day stamp. You can do that 2 times per year and you can extend each one by 30 days. That's at least 120 extra days on top of doing one more by air.

If you're planning on doing the ED visa conversion in Thailand you'll need to do it very early into one of your entries as it can take weeks, and you can only apply at immigration if you have at least 21 days left.