Then what stopping you?
What I did previously for my condo, I hired an ID firm that design the overall and supervise the construction. But I also sourced for Ikea kitchen and their installer, SPC flooring with the installer, windows and sliding door with their installer, kitchen hob/hood with their installer. Coz if issue arised later with those stuff, I can directly goes to the individual source, rather than talking to the ID firm, easier for both of us. Also, I goes for loose item/furniture and minimize carpentry works as much as possible. So mostly kinda 'plug and play'.
So my main reason to hire an ID firm is for the design, prepare measurement and the needed finishing for the other installer to work on, and to supervise all the work.