Several red flags with your plan.

First, plywood/spar(marine) varnish for a horizontal surface is about the worst combination possible. Not only would that combo need recoating almost yearly, it will warp within 12 months or after the spring rains.

Solution? Use exterior ply for the top and overlay tile with waterproof grout. Or use metal sheeting(a single piece) bent over the edges. Or concrete cast in place. Or another water proof idea.

That means a 2x3 constructed frame is too light weight. Needs to be 2x4 with bracing/construction sufficient to support whichever top you choose.

As long as you install the siding properly, regular construction lumber is fine---your(or a neighbor's) house is built the same way.

Contact with the ground. Does that mean concrete patio? Or does that mean dirt? Two completely different means of construction. Concrete patio installation could be PT lumber base installed with Tapcon screws(using a hammer drill for the necessary holes). Or powder actuated nails.

In dirt means digging piers to the frost line and a much different base design.

It may sound as if I am making the project unnecessarily complicated, but long lasting construction has to be done so the structure will withstand nature.

Handymac, thank you so much. Your input and guidance is exactly what I am looking for! I'm fine with however complicated this gets, just really don't want to put time and effort into a project done incorrectly.

The bar will be placed on brick pavers. I like the idea of tile for the top very much. Do you see any problem with finishing the cedar siding with the marine varnish or should we consider another option? Thanks again!

Cedar will naturally gray if left unfinished(as will several mother wood species used for siding/outdoor use.

The addition of a finish, like spar varnish(with UV protectant additives) will retain the color(natural or stained/dyed) longer. The spar finish will degrade and need to be refinished periodically---probably every 2-3 years. Light sanding(difficult with rough cedar) and recoating is the usual procedure).

A better idea might be painting with transparent paint. (I'll wait as bit for you to digest that statement).

Most paint companies have several tint base formulations. Because the wide range of colors requires different formulas.

I am making a walnut cross(8' tall) and applying Sherwin Williams Ultra deep tint base without any color added. The base dries clear.

That surprised the SW store manager. He had to try it.

Paint protects better than varnish/etc.