How do i figure out a schedule? Should i change the hours we have the venue for?

I would ask the photographer as mine was helpful in regards to timing including the ceremony on what not. I guess to answer your exact questions see my responses next to them:

The thing is, I have no idea how to set it up? Depends on your style and decorations you want. I would get help on this maybe an outside place or see if family will help. I would do that as soon as you are able to get on the premises.

What time should the ceremony start if I only have until 8pm? Ask the photographer but depends on how long you want the ceremony to be. I would say maybe 3 the latest for time for cocktail hour and for the reception. Maybe even 2 but not a lot of time to decorate.

Is this the time I put on the invites or do I put down an earlier time for cocktails so everyone makes it before the ceremony starts? The time you put is the time the ceremony itself starts and maybe the ending time.

Is daylight going to ruin all of the ideas I have about making it feel like a cozy holiday party? Depends on your set up and where the event will take place. Daylight will happen earlier but to me that should not factor how you make it a holiday party such as decor, food, drinks etc...

Should I see if we can have the venue until later to preserve the low-key holiday theme? If your budget allows. I am not sure your full concept. Do you want dancing or games. I mean if it is a low key wedding you may not need all that time but if you want a full party style reception afterwards you may want more time.
Also, if people are worried about driving in the dark, why not stay at the bnb that is AT the ceremony site? Is this inconsiderate of me? Do not worry about how people travel. They are adults and that is on them. You tell them the time and place and they can make their own decisions how late they stay.

When we booked ours, we weren’t sure as to what time we should have the ceremony. We’re getting married on 4/3/21 in Mesa AZ. After talking to both the photographer & the owner of the venue we figured 4pm with a 30 ceremony. That gives us the afternoon/sunset lighting for pictures. I would start with your photographer & the rest will eventually fall into place.

If you have a set block of time for your event, that is what you work with. Have the ceremony around 2 with reception following. Your photographer will know how to work with lighting. Lots of parties take place during daytime and it doesn't change people's moods as far as whether they're festive or not. If you're enjoying yourself, they will too.
The time you walk down the aisle goes on invites. They know to arrive up to 30 mins prior to be seated. Cocktails generally are not served before the ceremony, but if you do, mention it on the invites.
Guests will figure out their own travel plans.

Is the 12-8 including setup and tear down time? Because you also need to take that into consideration.
We have 4 hours before our ceremony to set up and get ready and then 5 hours of event time (ceremony and reception).