CMYK add-ons

Screen printing is a niche/specialised area of printing so it is no surprise that it will need to be treated as such, particularly with the generation of printing white and spot colours. All of this is possible in Photoshop with the right knowledge of the tools and the end process. $500 is cheap for a specialised professional tool that likely saves you so much more time/money over using alternative hacks. All trades have certain costs to do business.

If you don't have the appropriate 3rd party ICC profile creation software, I would suggest that you look into the pre-ICC profile colour separation method of Photoshop, the legacy "Custom CMYK" with a GCR using maximum black generation. You can include this into an action and include selective colour, curves and other steps to fine-tune the result and decontaminate "pure" primary colours (which could happen pre/post separation). Another method is to use "chops" or "channel operations" on either the RGB or CMYK data using the apply image command.

Same thing. Well, that's what SWOP (Coated) does: this profile makes rich blacks, it makes colours suitable for printing with SWOP (Coated) inks and papers. All separators, given a SWOP profile, should produce the same results, pretty much. But clearly silk screen tee shirts are not a SWOP (Coated) printing press, you need to use the right profile.

If you want to change the CMYK conversion, you do it by getting a new profile. You don't need to spend $500 on a separator. You, might, however, need to spend much more than $500 on specialist profile creation software.

Whether this is "normal" depends on the CMYK profile you selected. How do you select a profile? Have you looked for a profile with the properties you want?

Are you saying your black comes out on all seps (rich  black) and you don't want rich black? Or it only comes out on the K plate and you want a rich black on all plates? I don't think you need a different separator, but you need to look at your colour profiles and colour usage. Do you work in RGB or CMYK? What CMYK profile do you use, and how did you choose it?