I try to do similar. I'll even break down parts of the plan into separate docs and have it solely refer to them one by one as needed with /clear between. It's a more manual process but not the worst. I could probably even whip up some kind of skill to automate it somewhat with subagents or similar.

You can just tell your main agents "I have these implementation docs in <DIRECTORY>, please spawn parallel subagents to implement all of them" and I generally find that works, and what's great is it's in parallel too so takes less wall clock time (even if there is slight token duplication from having the main agent and the subagent both read the prompt/doc, but overall I think it is less usage of tokens unless you go crazy with /clear).