The problem is that you don't know in advance what the 20 steps will be, and you may spend time solving the wrong problem.

Make step 1 as simple as possible, step 1 is small and changes are cheap, you can even rewrite everything with little effort, no need to prepare for anything at this point.

Step 2-4 will inevitably break something, no big deal, rewrite the parts that need to be rewritten, it is still cheap.

At step 5 rewrites are starting to get costly, but now you have a better idea of what is here to stay, so it is now time to invest in things like regression testing, style guides, etc...

At step 10, you have quite a history, rewrites become a really bad idea, no turning back after that. So now it is time to think about the future. The difference from doing that in step 1 is that you now have 10 steps of experience and only need to look ahead 10 steps.