Need to reserve busses for kawaguchiko?

Hi, we am going from Shinjuku terminal to Kawaguchiko Station by highway bus on Monday 8 Apr, returning on Friday 12 Apr. I am not sure if reserving a bus ticket is required, or can I just show up and buy one?
We plan to travel in the afternoon both ways. I heard the early morning busses tend to sell out first and weekday afternoon busses should be plentiful, but not sure if that is still accurate, especially since it will be cherry blossom season.
When travelling, in general I prefer not to book things in advance because plans can change and it is nice to have the flexibility, but if I were to book, can the time be changed later?

The booking site I found: https://www.highwaybus.com/gp/inbound/index

If there are no busses to take me either way, is a reservation required for the train?
Don't want to end up stranded and having to make a very expensive taxi trip. However, in that line of thought, are there any charter taxis that will cost less, say ~25,000 yen or less each way? For a taxi full of people that may actually make sense.
Thanks, just considering my options.

Staying a week in Kawaguchiko you might want to rent a car. What are you going to do all that time there?

I can�ft speak to the busses but for train you don�ft need reservation IF you don�ft take the express. The local trains are non reserved only.

Thanks, for the input. Do the express trains tend to sell out or is that like limited-express where you usually just buy it at the station?
We are not driving but our hotel/resort provides pickup and drop off from Kawaguchiko Station.
None of us are comfortable with long distance driving and we don't need a car while there.
Aside from the 2 travelling days where we will just be relaxing in the resort, for the 3 full days there we intend to spend a day at Fuji Q Highland, another day with the lake cruise and ropeway plus bus or taxi to the wind cave. The last day will be taking a bus to the 5th station of Mt Fuji, not climbing it and it is not climbing season anyway, but just to take a look and walk around the 5th station. If weather is bad or if for some reason we can't go to Mt Fuji then we will just relax at the resort and walk around town instead.

The express trains DO sell out. At least the one time I wanted to take it (weekend) it was sold out already a few days before. I wanted an early morning train.
Instead I took the local trains.

we will just be relaxing in the resort, for the 3 full days there we intend to spend a day at Fuji Q Highland, another day with the lake cruise and ropeway plus bus or taxi to the wind cave. The last day will be taking a bus to the 5th station of Mt Fuji, not climbing it
A day at FujiQ I understand, but the "lake cruise and ropeway I assume you mean Hakone area which is not Kawaguciko. As for 5th station, that takes five minutes unless you want to spend lots of time at the tourist trap shops - when I visited there was nothing to do/nothing to see - one of the most boring places I have ever visited in Japan. I took photos of the parking lot - that was less than dull.

Hi JapanCustomTours, I meant the Lake Kawaguchiko Sightseeing Boat, and the ropeway next to it.
During that day we also intend to walk around the lake area and do sakura viewing, including from the north shore.
As for Mt Fuji, if the trails are open I might walk for about 30min then turn back. There are also observation decks etc, but mainly because we are in area and we would like to be on the famous Mt Fuji for the memories and photos. Seems shallow to do it just for the sake of that I know, but we are going to that area, everyone back home expects us to get on Mt Fuji lol. It should not take long, so the rest of that day will be relaxing at the onsen resort, and walking around town. We booked a luxury resort for 4 nights for Kawaguchiko, the most expensive per night of the trip.

So I am worried if I do not book the bus, will we be stranded in Tokyo, or possibly stranded there?
Will the local trains accomodate our luggage, and does it take significantly longer?
Our original plan was to take a short taxi ride to Shinjuku bus terminal and take the bus to Kawaguchiko station, and the hotel provides shuttle to and from Kawaguchiko station. Return using the bus back to Shinjuku bus terminal and take a taxi to another nearby hotel.

Definitely book the bus.

Was there on Jan 27 and took the bus from Tokyo station and next day from Kawaguchi to Shinjuku station.

Both days bus was full and the bus to Shinjuku on Sunday afternoon was sold out and the bus company told people to take the train.

The station was packed so I think lots of people will be standing in the train all the way back to Tokyo.

@Jamesc
Thank you, I will definitely try and book the bus then, better be safe than sorry.
I guess if I arrive significantly earlier and there are empty spots they may be willing to let me take an earlier bus, but from what you are describing it is quite unlikely there will be empty spots.

May I ask how our luggage will be handled, will it fit on the bus, do they have a luggage compartment? Total two large suitcases that fall under the carry on size allowance of shinkansen without needing to reserve luggage space on the train, but not sure if bus will allow.

Had the same problem.

Arrived early morning at Narita, took the bus to Tokyo station and needed to collect JR passes and book saet reservation.

Made the decision to do bus seat reservation but worried about flight delay and how long to collect the JR passes.

Anyway when we saw the queue thought right decision to reserve bus seat tickets.

Bags are no problem. They have a very large luggage bay under the bus.

Thanks James, may I know which bus company you went with, and/or which you believe is the best? Or is there only one?
Not sure if this is the right one: https://www.highwaybus.com/gp/inbound/index

This is the one!

https://www.highwaybus.com/gp/inbound/index.