Baseboards at big box stores are typically standard profiles and can be found in wood, MDF, or PVC. Can you link your exact profile?

I'd bump up the sizing, and stop worrying. A PLAIN 1 x 6 is 5 3/4 and a plain 1 x 4 is 3/3/4 both eight foot lengths. All plain, all same "profile" as it's just square : )

Beefier!!! Unless you have very low ceilings and limited window /door casing space.

I normally skip shoe moldings and have them scribe to the wood floor.

Here's that size in a bath

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1 x6 at floor, 1 4 at windows below

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all through, and everywhere the same.

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Or skip the wood/MDF trim and use a coordinating tile for the baseboard.

We have MDF in our bathroom. It's held up. As mentioned, back prime all surfaces and cut ends.

Typically, 1 x 6 is 3/4 x 5 1/2 and a 1 x 4 is 3/4 x 3 1/2. There are also 1 x 5 boards that are 3/4 x 4 1/2.

The problem is the thickness. Flat stock baseboards are usually between 7/16“ and 9/16“ while s4s boards are typically 3/4”. That little bit of extra thickness can change the look significantly especially given the viewing angle of baseboards.

ETA: Just to be perfectly clear… 1/2” flat stock baseboard can be contemporary/transitional but 3/4” is going to look Craftsman. If that fits then great, but we don’t know that.

Exactly why I love it.

We had terrible experiences with MDF baseboards in our kitchen and bathroom, though a PO installed it so no idea how high quality it was. It was painted in high quality paint (BM oil-based Satin Impervo) but still got swollen when it got a little wet in passing (as it often did in both rooms). By the time we replaced it in the kitchen, there were areas where chunks of it had disintegrated, and we still have that issue in the bathroom--hope to redo that room someday soon. I'll never use it again as a result of that experience, though I imagine newer MDF may well be more moisture resistant. Most baseboard profiles we looked at for a recent bathroom project came in both wood and MDF, though, so we chose the wood. I like the idea of a tile baseboard if the floor is tiled, too. (FWIW, we opted not to use the same size baseboard in our bathroom anyway--most of the house has 7" baseboards which looked overwhelming in the bathroom, so we did 4.5" instead since they aligned with the height of the shower tile. I don't think anyone notices!)

We have MDF trim in our whole house bathrooms all 3 of them. This has been in our home for 16 yrs not one issue They are painted well and we have good venting which BTW in a bathroom.you need no matter what you choose for trim . I have never had a client who had the issues listed above so can't comment on that.