Tax rate select missing from product price in commerce 8
Try these 3 references:

1) Need to modify tax type based on user field in Drupal 8 Commerce 2

Here you can see an example of creating your own service in a custom module to interact with the default TaxOrderProcessor or writing a simple module to override the tax order processor service at all.

2) https://www.flocondetoile.fr/blog/apply-vat-rate-product-drupal-commerce-2

Here you can see how "to vary an applicable VAT rate according to a product, or a product type, we just need to implement a service that will declare the trade_tax.tax_rate_resolver tag with a priority at least higher than the default TaxResolver provided by Drupal Commerce."

3) https://www.rapiddg.com/blog/modifying-product-prices-drupal-commerce-2-drupal-8

Here you can see another example of application of "resolvers", the concept that do the work done bu Rules at Commerce 1, which is the key of the previous comment (link 2).

Ps: At the moment, poor documentation about this stuff, and no UI to manage this BASIC feature :-(