11/16" works out to 17.5mm and although that isn't beefy for a table of this size it doesn't seem too thin to me.
Many tables have been made with tops are under 1" (25mm) thick, although the ones I'm thinking of are made from hardwoods which can be significantly stronger than SPF. But this is being very generic, which unfortunately the descriptor "SPF" is too as I'm sure you know. Obviously to begin you can't be certain what species you're buying in the first place and all wood does naturally vary anyway. More than that though modern 2x lumber is known for not being particularly high quality and varying a lot at the point of purchase — not hard to find, at the same store, good bad and indifferent pieces.
Obviously I can't provide any guarantees here but I'd say this will probably be OK. You should make every effort to carefully select the pieces you intend to join to make up the tabletop though. And in case it needs to be stressed, joint the edges well, weak joints will do a lot to undermine the strength of such a tabletop. Jointed properly however and it'll be like it's a single piece of wood.
I would be careful about how you move the table with the base attached, but that should be standard advice with any large table. With a sufficient number of fasteners you should be able to safely lift a table of significant weight regardless of the tabletop material, even particleboard/chipboard which is notorious for not holding screws well*. Just don't expect that it can be levered off the floor by one person and you should be fine.
A little further reading from previous Q&As if needed:
What's the best way to widen a butcherblock countertop?
Can clamps be too tight?
Methods of jointing without a jointer
*This isn't theorising here, I'm basing this on handling much office furniture which over here invariably uses chipboard for the tops. I've never broken a top free from the base of a heavy piece, although I have seen the damaged tops where this happened, in every case I can think of it looked like too few screws were used to attach the base to the top (as few as four, not nearly enough for security with a heavy steel-framed desk).