Your local bike shop is correct, you're up to the whim of whatever is available. My buddy at SRAM noted in June 2021, they're already sold out through 2022 in components.

Something to consider is that Tiagra is 10s while 105 is 11s; Tiagra is also heavier. As far as performance, they're very close. However if you can swing it, I'd go with at least 105 or Ultegra to keep with 11s. If you're a avid enthusiast, I think you'd appreciate the smoothness and reliability of Ultegra. 105 is great as well but I notice it isn't as smooth and light in action.