How can you add steps to a workflow execution later?

I’d like to be able to implement just 1-2 workflows at a time.

What’s the best way to build it so that I can add steps to the workflow, and they’ll automatically execute from the previous run when I update the code and implement.

Was thinking of always having a failing subworkflow at the end, so the main workflow will never be considered complete.

That way I could always add steps.

Is there a better way to do this?

Not sure I fully understand your requirements from description but yeah,
sounds as you want to keep the execution in running state after completion of all current milestones.
This execution can await a signal that triggers it to ContinueAsNew
passing in its current state to next.
Temporal worker versioning also supports this use case, (without signal trigger) see here if helps.