Day Trip to Nakatsu Gorge from Kochi

I'm considering doing a day trip to Nakatsu Gorge from Kochi in a group of 5 next month.
I'm not too confident with taking a bus after looking at the bus schedules.
I haven't been able to find any websites that offer half day taxi tour from Kochi.
Is it easy to book a taxi in Kochi on the day or 1 day beforehand?
How much time and cost should I prepare if we are focusing only on Nakatsu Gorge?

I found a tour from Kochi city.
https://www.veltra.com/jp/japan/kochi/a/156495
This is the URL of the taxi company that holds the tour.
https://www.fukuitaxi.com/kanko.htm
If you have any questions, please click "お問合せメール"（Email us）.
The tour below seems to be available in English as well.
Time is 60 minutes.
If you contact the tour company here, they may be able to tell you about taxis.
https://reserva.be/niyodoblue
It seems that the number of buses from JR Sagawa Station is certainly small.
0 on holidays and 2 on Sundays.
https://www.yunomori.jp/access.php
On this page, it is written that when using a taxi, please make a reservation before leaving Sagawa Station because the number of taxis is small.
https://www.niyodogawa.tv/niyopress/wp-content/uploads/2019/10/c003430...