I'll give you my experience of the temple and Koyasan.
For this I wanted the whole experience, so I chose one of the pricier places that had its own hot spring, and I did the overnight since I wanted to see the cemetery at night as well. I really didn't know what to expect but I wanted to see the more spiritual side of Japan.
The first observation that I made was that over 90% of the activities going on were regarding housing the guests like me and taking care of the meals. The place I stayed had over 100 guests staying there that night (on a weekday no less) and for whatever reason there were a huge number of French tourists there. The temple was nice enough - it had 2 nice gardens, plus a small koi pond and some beautiful art work in the temple. But it was a long trip to reach Koyasan, and I was getting tired that night. I was still hoping to go see the cemetery but I was surprised to find that like a number of other temples, they had a 9 PM curfew. Had I known, I would have picked another place.
I recommend doing an overnight at Koyasan though, given the time and trouble it takes to reach there. But if you would like to see the Okunoin at night, you should pick a place with no curfew!
https://awesome-tours.jp/en/tour/night-tour-return-bus/There are paid tours there but you can just go yourself. And having a clock ticking away in the back of your head that you only have so many minutes left before you have to leave is not an enjoyable way to see the place.
So in the end, the temple stay was not really different from any ryokan stay - except the food was Buddhist vegetarian. As for visiting temples, there are what is shown on this site:
https://www.japan-guide.com/e/e4900.htmlbut if you mean visit the places with temple stays, I never saw anyone do that - the temples were open to guests staying overnight only.
So what's the guesthouse like? As far I could tell, similar to staying at any other guesthouse in Japan.
Was it worth it? Yes - Okunoin makes it worth it. I could have spent a whole day walking through there - it was marvelous.
But if I couldn't go to Okunoin while visiting Koyasan,
forget it.