RO water out of the tap, what are best products for conditioning and restoring kh?

To remove anything from the water, yes please do it gradually with partial water changes.
10% partial water changes is a little too slow. You can do 20~30% and it is still considered as small water changes. Just no more than 50% each time would be safe enough.

Carbon filter will not affect the cycling as it will not reduce ammonia by any noticeable amount. I left mine in my canister filter when I did fishless cycling.

Just use a single product to restore water buffer/mineral content is enough. Make sure you read the label carefully. It is important. You might as well use the same water for partial water change for your betta's temporary home. So he get a head start to adapt to the new water.