Good to see you are wise young(er) man.

#1... long term loans are kind of like being shot and leaking blood,,,  it's not bad unless you are unable to make more blood than you leak out.

If you can "bump" the payments along and cut down that long term process you'll be farther ahead.  The two tricks are, can you pay ahead and not run afoul of the land contract under the young farmer rules ??  And number two, can you actually 'rat hole' that money *saved* by paying ahead on the land contract?  Not saying to put it in to a coffee can in the back yard, but actually keep track of the money saved and specifying where it is exactly. (or at least approximately)

 #2.  Money in a bank isn't a real 'barnburner' investment from the interest standpoint. I do understand it's cheaper to use one's own money for purchasing input costs, but,, since you have a good ratio,, yes "short term money" is more dollars in interest but under the 1st senerio, it sounds like you are producing alot more 'blood' than you are leaking out.

#3. Is the Trickiest option,, can be great or great disaster. As this is your first year, I'd
     get through the year FIRST.  I'd be always watching-learning-thinking,, "could I start
     "this" business, make it sucessful and still be working for my relatives right now?" 
      Whatever the sideline business is,, it  would have to be flexible enough to allow you
      to schedule the side business along with working for the relatives, right?
      ( for a side business if it were me,, I'd look at 2 angles, first build on your "strengths"
       ie:   you were in the banking business, what jobs are out there the would benefit 
      from that expertise?? Consulting, building spread sheets, etc???  low input cost 
      you've already got the major tools for that job.  Or,, a more manual and seasonal
      sideline business like spray foaming. There's been lots of posts on this site
      about guys wanting or looking for someone who could spray foam insulate buildings
      etc. Alot of that business would be in the summer when warm, yet (hopefully) not
      interfere with your main job with the relatives.)

I would lean toward #1 if it's possible, as you say, you can 'tap the resourse' if needed in the future.  #3. would be second choice after you get a year of experience. #2 is kind of the last option by default.

Probably zero help........