Yeah it’s always tough to find the best option on a budget. Each one will have good and bad sides.
So I don’t coach anyone who recently has used the Garmin pedals. However I have in the past and they always seemed to have consistency issues. I know the pedals use to have to be torqued correctly which was an issue. Also a pedal is something that will wear out over time as opposed to a crank arm or crank spider.
As for single or dual, single sided won’t be as accurate but consistency should be there which is the most important thing. As long as your one leg is always working at the same percentage compared to the other it shouldn’t be much of an issue.
A power tap is really consistent and reliable. So you may notice a difference in your power and FTP with a new system. Also the power tap uses power coming from the rear hub which is different than a pedal or crank based system.
For one single sided based power meters Stages as improved a bit over the years and I coach many who use those pretty successfully. I would recommend using something such as a Quarq. It might be a bit of investment at first but I’ve had the same one for several years with no issue. Also with being a FasCat Athlete you can get a possible discount through ExcelSports.