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

It is fine.
would recommend you to use as few products as possible for
1. Less trouble to yourself.
2. Lower chance for error.

Remember if you have to mix several different types of water, and throw in a few different products to alter the water hardness in mix every time you change water, there is always room for error. For example you could have put in the wrong amount for one thing, and it might all be messed up. I'd keep things as simple as possible.

Don't bother with shells, rocks, bones, whatever lol. They are for making water harder over a long period of time. Betta likes soft water anyway. The less you mess with water hardness, the better. By using a single product, you can easily make sure your betta get the same water every time you do water change.