Every writer I respect is a reader first and foremost, and to try and write without reading widely is nonsensical. If he were in court, arguing to a jury about why he did the things he did in the novel, what would he say? If so, can I make it better? A car crashes through the wall.
With rare exceptions, simple no longer sells. Is your protagonist all good? True, to an extent And you have to read. What motivates the person — as an individual and as a professional? Your plot needs intricacy and a surprising number of well-planned, well-executed twists.
Then choose the best one. Standout thrillers need complexity and webs of conflict, so that every page hums with tension. And that takes practice, trial and error, being able to take it on the chin when a good editor, or reader, or fellow writer, says no, try again.
What you do there can be very varied, but cute, cosy crime is a very limited market now. Be complex. Some of the people you knew there have changed their names.