For private projects, and early fixed pricing, use a modified design build or design assist (= design build but with a formal tender). Spec loads, performance, preliminary sizing and connections, etc. and go out for a price at that point.
Image c/o: KK Law
Advice
Example of a mainly successful project: Brock Commons — involved a high degree of collaboration: Integrated Design Process — design offered 3-4 options for every connection, which every stakeholder vetted — invested in building a mock-up, where many problems were solved in advance of construction — the design worked with standard panel sizes
Advice
Tolerances are different for each different trade. Mitigate potential problems with communication in the procurement phase.