I am also noticing that I have to set them off on a hyzer line to keep going straight.
This is pretty typical. With an average neutral putter mold, you'll probably have to incorporate a bit of hyzer past 200-250', and especially at longer distances. It does depend on the mold though, some of them don't require hyzer. Which putters are you throwing?
As I get more powerful and better technique will I need to alter my form to keep them straight? Should I switch to midranges when this happens?
Distance comes from timing and acceleration rather than power.
Again it'll depend on the mold, and the distances you're throwing.
Are you trying to rip a Nova 350'? You'll need to give it some hyzer to keep it straight.
Are you throwing an Envy 250'? It doesn't need any hyzer.
Are you trying to throw a Pig 400'? You'll need some anhyzer
There is no point (That I've discovered) where you're throwing putters too hard and have to switch to mids. I throw Proxies 400-450' and they're neutral. Mids are there for more distance in general, punching through wind better than putters, low ceilings, stuff like that. There is nothing about a putter that makes it incapable of handling a full powered throw, and in fact our putters were once what people considered distance drivers.