WordPress/WooCommerce developer to fix, speed up and extend our existing store

We sell specialist equipment on a WooCommerce store that has been running for about three years. It works, but it has been patched by four different people in that time and it is starting to show. Immediate problems: - Checkout intermittently fails on mobile Safari. We do not have a reliable way to reproduce it, we just see abandoned orders. - The product filter returns wrong results when two filters are combined. - Order confirmation emails sometimes do not send. We only find out when a customer emails asking where their order is. - The site has gone from about two seconds to six on mobile over the last year and we do not know what changed. - We have 34 plugins. We suspect a number of them are doing nothing and some may be conflicting. Then, once that is stable, there is one thing we have not been able to buy off the shelf. Customers who order certain products need to book a fitting appointment at one of our three locations, and that booking needs to be tied to their order and visible to the right branch. Every plugin we have tried either does not quite do it or costs more per month than it is worth to us. What we are looking for: - Someone who will work inside what we already have rather than proposing to rebuild it. We do not have the budget or the appetite. - Real PHP, not only page builders. The booking piece will need actual code and we would rather one person did all of it. - Fast to respond. When checkout breaks we need someone that day. - Someone who tells us when we are asking for the wrong thing. This starts as a fixed-price piece of work, but we would like to find someone to keep. and would rather stop doing that. To apply, tell us two things: what you would look at first for the checkout issue, and one problem you have fixed that turned out to be caused by something other than what the client thought it was.