Reply To: Deciding between two primary keywords

In my opinion you are overthinking it. Since you are a plumber and you offer your service on the site, you should go for keyword plumber. If you had website about plumbing, for example with some guides etc., It would make sense to go for a plumbing.
Anyway do not focus strictly on keywords, focus on your target group and their search intent. Imagine what they want to see in SERP.