Can I refund a game that I have a decent chunk of playtime on and then buy it again because it's on sale?

Did you buy it less than 30 days ago? If so, then yes.

dont think so
you already played it much and they will probably ask why you want to refund it

I concur. Put in a ticket if it's under 30 days old; You might mention you just want the difference in sale/bought price and you'd take wallet funds and they might do that instead of doing a refund/rebuy thing...

This seems like the best course of action.

GOG allows you to refund a game within 30 days for any reason. Although they have the right to deny refunds from people who appear to be abusing this privilege. So pick your battles carefully. A large discount a few days after purchase may be worth pursuing, but requesting refunds anytime any prior purchase goes on sale will quickly lead to a "this is the last time we're doing this for you" response.

Also keep in mind that refunds are currently taking a lot longer to process. If you're willing to accept GOG wallet funds, those requests seem to be going through very quickly (typically a few days; some people have even reported same-day refunds). But if you want a refund back to your original payment method, there are incidents of people waiting over 30 days for those requests to complete, in which case whatever sale you're trying to get will have long since expired.

TL:DR: You can request a refund for any reason within 30 days of purchase. But refund requests can take time and the sale may end before you get your refund. And frequent refund requests will come under scrutiny, so pick your battles carefully.