Hi, and welcome to the forum!!
The test kit that we recommend for all pools is the K-2006. Take a look at the link in my sig--it can be bought online from different places, but at a good price through Amazon. Look for Amato industries as the seller, because they seem to have the best price so far--arond $60. This kit will measure anything you need to measure, including accurate FC/CC levels. The only thing it won't test for is salt or metals, but there are standalone kits that can be purchased with it to test for those if you need it. The OTO kit you have will work for daily "snapshots" of your water, but the better kit is going to be necessary to get the pool balanced and keep it that way.
I don't know about the modifications you want to make to the ABP, but somebody should chime in here shortly with opinions. The pump/filter on those pools is typically mismatched, so a bigger filter is always going to better, even if you can't add additional returns.
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