There is a better way to handle this than the one you are currently wrestling with. The real issue here is not the data itself, but the assumption that you need to manually combine weekly tables into a single monthly view. You do not. The solution is not a complex Power Query formula that keeps throwing errors. It is a structural change in how you lay out your spreadsheet from the start.
The problem you are facing is common, and it stems from creating a new table for each week. That approach forces you to reconcile multiple sources later, which is exactly where the errors and frustration creep in. You are not bad at Excel. You are fighting against a design that was never built for this kind of ongoing tracking. The fix is to consolidate all vehicle usage data into one continuous table, with a column for the week number, the department, and the vehicle. Then, a simple pivot table or a SUMIFS formula can give you the daily, weekly, or monthly totals you need without ever merging files again.
What this means for you is that the answer is not in the query editor, but in your data entry habits. If you keep one running log of every vehicle use, with a date column and a department column, the aggregation becomes automatic. You can then group by week or month using a pivot table, which takes less than a minute to set up and requires no scripting. The reason Power Query keeps failing you is likely because it is trying to append tables that have mismatched headers or extra formatting, which is a symptom of the underlying layout problem, not a flaw in your ability to use the tool.
The practical takeaway is simple: stop building weekly tables and start building a single data source. That shift will save you hours of manual work and remove the error-prone merging process entirely. Your request for an automatic tracker is reasonable, but the first step is not a better formula. It is a better structure. Once you move to one table with a date column, the rest of what you want will fall into place, and you will wonder why you ever tried to piece together weekly snapshots in the first place.