How can you optimize JMeter script maintainability for a large, growing regression suite?

Modularize common flows - login, authentication token setup, shared navigation steps - into separate, reusable test fragments using Module Controllers or included test fragments, rather than copy-pasting the same sequence of samplers across dozens of test plans, so a change to the login flow only needs to be made in one place.

Externalize environment-specific and frequently-changing values (hostnames, credentials, thread counts) into properties passed via -J and read with __P(), rather than hardcoding them directly into the .jmx file, so the same test plan can run against different environments without needing per-environment copies of the file that inevitably drift out of sync with each other over time.

Adopt a consistent naming convention for samplers, variables, and thread groups across the whole suite, since a large suite maintained by multiple people becomes genuinely difficult to navigate and debug if similar elements are named inconsistently from one test plan to the next.

Store .jmx files (and supporting CSV data files) in version control alongside application code, treating test plan changes with the same review process as code changes, since a large regression suite that isn't versioned tends to accumulate untracked, undocumented drift as different people make ad hoc changes over time.

Periodically audit the suite for redundant or outdated samplers - assertions checking conditions that no longer apply, correlation logic for tokens the application no longer uses, samplers hitting deprecated endpoints - since a growing suite that's never pruned accumulates dead weight that slows execution and adds noise to results without adding any corresponding test value.