Courtright - you may well already know this, but for discounts, have you tried the price levels you can set in Product Control? There can be up to 6 of them. I use them for Retail, Wholesale, Local Discounts, Employee Discounts, Non-Profit pricing and Loyal Customer discounts.
These are all set per product, so you can tailor your entire inventory to vary by exactly what discount level you'd like. Plus, you can tailor them by either dollar amount or percentage, so you can really have a lot of control.
Also, once that's set up, all that needs to be done is to set the Price Level in Customer Info. You enter it once, and it stays there until you change it. For people like veterans, for instance, it works great.
Also, when a customer with such a discount is brought up for a transaction, a warning pops up that the prices are being modified from standard retail, so the clerk will be aware of the discount.
AND, you get a note on the receipt that tells the customer how much money they saved, just as with other discounts.
We use those discount buttons only for one-time discounts, e.g., when a customer has been negotiating a lower price. I haven't been able to get them to do quite what I want.
For Price Levels, it'll seem daunting at first to put values into all these fields. You don't want to do it right in EZP - just do a product export to Excel, modify them en masse there, and then import them with the new values.
I can't speak to the sales tax aspect, since we don't have it here in Oregon.