Back in late 2019 we used a Chase free night cert at IC Beppu. The point rate was 80K, while the cash rate was around $600 for the Intercontinental Room (now called Classic Room I assume). I think for 80K points it was fair value.
It was a one-night layover between flights so we did not rent a car and relied on the hotel shuttle. If you will be staying more than one night and want to use IC Beppu as a base to explore the surrounding area, I would recommend getting a car. Although I don’t see a reason to stay more than a night or two.
If you do make it to Yufuin, I can’t recommend
Kamenoi Bessie enough either for dining or staying a night.
KAI Aso is also on our list the next time we visit Kyushu - can’t wait for Japan to open up to us