Question for yourself is - How 'competitive' would I like to be? If the answer is 'somewhat' and you want gearing more suitable for BOTH road and dirt road/gravel, then 2x gives the best of both worlds. However, in the world of average (but confirmed) 60+ cyclists, 1x or 2x isn't going to make that much of a difference...
IF you already paid in Full for the bike - that's a first mistake (not knowing what the current convention is - I would give a solid deposit but never again full pay on anything 'custom').
You sound to be too nice, too mellow, someone who rarely complains ... there's a point where embarrassment (of the LBS AND supplier) is necessary to get some completion.
If it's about 'parts' - Shimano is good stuff, but so is SRAM and other component makers, like Praxis. If the frame is ready, there should be a way to get it completed with equal level parts.
If you can only have a 31 on a 2x, then you can get a cassette which goes to 36 - But I would start with the 34, because the center of the cog set will be much more likeable for the vast majority of road (and avg dirt road) riding. if you find the 34 is not enough, you can always go with a 11-36...
You're not making 'compromises', you're opening options to get where you want to start (riding).
Many focus on what they 'Didn't or weren't able to get'. When what they have is eminently suitable for what they wish to ride.
The key is learning about and enjoying what you do have.
But get the bike !
If the LBS hasn't bothered to suggest alternatives, then they're not doing their job.
Whatever you eventually come up with will be light years more enjoyable than the old gaspipe... Which can still be loved for it sturdiness and memories...
... you're not getting any younger... LOL! (coming from a 73 yr old).
Ride On
Yuri