I can’t say what will work best, but I can speak to proportions on one of the mixes....
For the spar/ linseed/ mineral spirits thing, at least as it pertains to use in other types of woodworking projects....the customary mix is roughly 1/3 each. There’s plenty of room for modification of that recipe, it will just impact the level of protection, penetration, dry time, etc. You can also play around with adding oil based pigment (such as oil paint from hobby store). In my limited experience, a little bit of pigment goes a very long way.
Red oak is pretty porous and prone to rot, but lots of folks use it for beds. I like Alvin’s suggestion of treating the ends VERY well.
A couple other thoughts:
1) I assume the sawmill wood was stored and dried for adequate time before dimensioning it down for the bed boards?
2) Would be a very unique look, perhaps not your thing, but if you’re looking for even more protection....do a web search for “Shou Sugi Ban.” Basically ancient Japanese method of charring wood to preserve it. Wood can apparently last centuries when it’s charred this way.
