You are correct that WTFINL is the appropriate weight for most analyses of basic monthly survey (BMS) samples. Without more information on your analysis, I cannot provide exact troubleshooting, but I will share some information that may be helpful.
Most importantly, if you are using monthly survey data, but tabulating frequencies for an entire year, you will get an estimate that is approximately 12 times larger than is correct. The monthly CPS is a snapshot of the U.S. population in that month. Monthly estimates are accurate on their own (keeping in mind that labor force variables and others have seasonal trends, and so February will not look the same as July, for instance). Another important consideration is that it isn’t generally possible to obtain accurate estimates when your sample size is very small. If the unweighted number of public school bus drivers each month is very small, your estimates may not be accurate. You may want to use estimates that pool data from multiple time periods, or analyze a larger group of workers (e.g., all bus drivers).
If you want to use the BMS to calculate an annual estimate of the number of bus drivers, you can divide WTFINL by 12. This would allow you to see how the number of bus drivers (averaged over 12 months) changes each year. If you want to use the BMS to calculate monthly estimates, you can tabulate your binary occupation indicator by month, rather than year. This would allow you to see how the number of bus drivers changes each month.
Also, note that the CPS is a panel survey. Sampled households are interviewed monthly for four months, followed by a period of eight months out of the survey, and then four more months of interviews. This design allows you to study the same households or individuals over time. You can read more about the CPS design and panel structure here.
You may want to consider the possibility that many school bus drivers do not work as school bus drivers during the summer months when school is not in session. This could affect your estimates if a much smaller number of people work as bus drivers during some months, lowering the annual average.
The Census Bureau’s occupation categorization scheme changes frequently to reflect changes in occupations over time. This means that OCC codes change over time. You should make sure you have identified the bus driver occupation code correctly in each of the samples you are using. The 2011-2019 data use a different OCC coding scheme from the 2020-onward data. The same is true of the variable IND; the categorization scheme and therefore the codes change over time.