Which food safe substance to coat the interior of planters boxes with?

"Pure" tung oil and linseed oil can take a month or more to cure.

Personally, if you want food safe, I'd use a 50-50 citrus solvent/tung oil from milk paint or Tried &True Danish Oil. I've used both and they are food safe. The citrus solvent in the tung oil makes it dry faster and much easier to apply. The Tried & True Danish Oil is a polymerized linseed oil that dries much faster and is food safe with no added crap.

Off the shelf Boiled linseed oil usually has mineral spirits and heavy metal crap in it to speed up cure time. Also don't use "Danish oil finish" or "tung oil finish" from brands like Varathane or Minwax or Watco. That stuff isn't pure at all and is mostly just mineral spirits with bullshite.

Kind of like how stores sell "juice drinks" that have little to no actual juice vrrsus 100% juice.

I love woodworking and making furniture and stuff and I have tried just about every finish available and every method of application. After years of trials, I use 50-50 milk paint tung or Tried & True Danish for anything that needs food safe. For general furniture, I've almost exclusively gone Osmo Polyx Oil in Pure.

Nothing is required for redwood or cedar. Wood planters not made of either is simply a terrible idea.

Not sure how you feel about the health of plastic but lining the inside of the box sure helps water less in the dry spells. Wood is constantly wicking away moisture. Just poke enough holes in bottom that it doesn't turn into a pool.

That being the case then marine spar varnish is the best I've found. But at least 6 coats. The majority of this type of coating are non toxic if fully cured.