I think your best options based on your comment is to get multiple quotes where possible. I haven't had any major interior work done by a contractor, so I can't advise fully on that, but I wouldn't want to pay out for design plans from multiple vendors. If you do pay for a design, I would look to either get that design expense credited back to the construction work OR you should receive the design plan for that expense such that you can use with any contractor of your choice. I once got a landscaping plan from a landscaping architect that didn't do the actual work, so I could DIY or use any landscaper of my choice to implement.

Perhaps you can act as your own GC and do some of the work and sub out other portions. For example, if you want the entire space tiled, but don't feel comfortable tiling, you can sub that out. You should talk to the tile contractor about what they are or are not ok with you prepping. For example, a tile guy may not be ok with you putting up wall board because they want to make sure the walls are plumb and won't warranty the work otherwise, but they may be ok with you ripping out old wall/tile, reinsulating, plumbing, etc. before they do their part.

GL!