Phnom Penh Sorya does that route - but the bus departs from PP at 8:15am (one bus per day). Another option would be hiring a car and driver - but that's going to cost a lot more than the bus. If you put Elephant Valley Project in the search bar up top you should find more information about the project, places to stay etc.
You can take the mini-van service from Kim Seng Express. They are good and safe. Sen Monorom is a very small place and there a few hotels to stay. The mini-van will drop you off at the beginning of the place and there will be moto-taxi boys to bring you anywhere. On the main street are 3 or 3 hotels & guesthouses.
I think there are mire than one bus going. I took the bus with another company than Sorya.
Also, unless the bus stop has moved in the last 1 1/2 years there is no need for an mototaxi, as you can easily walk in SM, and there will be people offering rooms who wil also give you a lift for free.
Regarding the hotel, I stayed at the guesthouse that belongs tonthe Greenhouse bar, it was comfortable and clean, and I paid around $10.
Just head to the Greenhouse bar, which is two houses away from the bus stop (on the right when looking at the stop). The guesthouse is literally 2 or 3 min away.
The pick up to the Elephant Valley project is also from the Greenhouse. You will enjoy it for sure, I loved it when I stayed there for three days 1 1/2 years ago. I would also recommend to stay an additional day in Sen Monorom to explore the surroundings. There is an absolute beautiful waterfall.
A word of caution about Kim Seng Express. I used this service today, from Sen Monorom to Kampot, with a change of van in Phnom Penh. The air conditioning was broken in the van from Phnom Penh to Kampot. Kim Seng Express was well aware of the fact before the van departed, because the driver was using a water hose in a futile attempt to cool the van down from the outside while all the passengers sat in the station. When I arrived at the station from Sen Mororom, the hood of the van to Kampot was open and a couple of people were tinkering with the engine. This is not a good omen. I've lived in Cambodia for five years, and drivers and office staff here are not mechanics. Before you commit to a ride like this, be sure as possible that things are in order. If there is any doubt you would be better choosing a later van or going with a differnt comany, even if you have to buy a second ticket. As the situation unfolded today, an apparently frustrated driver was also extremely rude to me when I asked about the problem with the AC. Ten minutes away from the station my clothes were soaked in sweat and the air in the van was dificult to breathe. I called back to the station to see if I could get a later van, but they said this was normal and it was an especially hot day. It was five degree hotter the previous week. You can't escape problems in Cambodia, but there are some precautions you can take to avoid obvious difficulty. I speak Khmer language well, but the driver first pretended not to understand me, and then started fiddling with the AC controls. Rude, frustrated, and not so smart - possibly a naturally concurrent set of qualities. Overall, I think this company has a good service, so my point is just to stay alert to signs and symptoms which might help to avoid a four hour ride in a furnace, which could quickly change your trip from fun to difficult. As of this writing Kim Seng Express has no reviews to comment on, so this will have to do for now. Good luck!
Kim Seng Express is a gamble. On my trip today with Kim Seng from Phnom Penh to Kampot, the air conditioner was broken. Ten minutes after departure my clothes were soaked and I was seriously concerned. In the current heat of Cambodia, a broken air conditioner is seriously dangerous. I asked the driver several times to open the windows because it was actually cooler outside than inside the van. To no avail, and calls back to the station met with pretense. There was no one interested in discussing the problem. My advice here is to get in the van and be sure the AC is working well in advance of departure. If it's not working, then ask the staff to remove your bags from the van, and get another ride. This is going to be an actual safety risk for some travelers!
Do not use this bus company!!!!! We made the Phnom Penh - Mundulkiri trip in 2017. The driver was driving crazy. He was really dangerous, was driving much too fast, sometimes on the left on the road in left curves where there was no visibility at all. He was horning all the time and didn't care about motorbike on the road. The owner of our guest house in Phnom Penh warned us before and told us that it wasn't a good company, way too dangerous. There are many accidents involving the vans.
A lot of people complain. If you care about your life, avoid it