Same or Different Models for Plan vs Build

How do you guys setup your models? Do you use the same model for plan vs build? Currently, I have

1. Plan - Opus 4.6 (CoPilot)
2. Build - Kimi K2.5/GLM-5 (OpenCode Go)

I have my subagents (explore, general, compaction, summary, title) to either Minimax 2.5 or Kimi 2.5

I have a few questions/concerns about my setup.

1. The one thing I'm worried about is Token usage with this setup (while I'm doing this to minimize tokens). When we switch from Plan to Build with a different model, are we doubling the token usage - if we were to stay with the same model, I figure we'd hit the cache? May not make a difference with co-pilot as that is more of a request count. But, maybe with providers like OpenCode Go

2. While I was uinsg Qwen on Alibaba (for build) in a similar setup, I seemed to be using up 1M tokens on a single request for the build - sometimes, half the request. I'm not sure if they are doing the counts correctly, but, I was not too bothered as it was coming from free tokens. Opencode stats was showing about 500k tokens used. But, even that was much higher than the tokens used for the plan (by about 5 times).

3. what would be the optimum way to maximise my copilot plan? Since, it's going by request count is there any advantage to setting a different model for the various subagents.

4. Is there a way to trigger a review phase right after the build - possibly in the same request plan (so that another request is not consumed)? In either case, it would be nice to have a review done automatically by Opus or GPT-5.3-Codex (esp if the code is going to be written by some other model).