If you’re interested in submitting an investigator-initiated R21 application through NIH Exploratory/Developmental Research Grant Program (Parent R21, Clinical Trial Not Allowed) but feel overly constrained by the budget and project period limitations, consider instead submitting an investigator-initiated R01 application through NIH Research Project Grant (Parent R01, Clinical Trial Not Allowed) for a 3- or 4-year project period. R01 grants are not required to have a 5-year project period.
But keep in mind, reviewers will expect you to have convincing preliminary data for an R01 application. Refer to Comparing Popular Research Project Grants—R01, R03, and R21. For exploratory research, your most competitive option may instead be to propose a refined version of your ideal project that fits within the parameters of the R21’s budget and project period limitations.