Negative funding on perps is a rare thing in a bull market. As a short term trade what you are doing might make sense, but you are facing a few big negatives/risks.

1. Funding rates might shift to the opposite on a dime.
2. You might not get a chance to re-enter with perps at the same or better price after selling on spot.
3. You pay fees on every trade you take (decent ones on spot trades).
4. Exchange risk that you mentioned yourself.