Is there an upsell app that delivers consistently and won't cause issues with my store?

There are several apps that have been designed with the seller in mind, a good one to use is the Upsellio app, by Expert Village Media. It allows you to be able to keep track of sales you have made and see what products are selling the best and the quickest.

Since most of the apps haven't worked for you, I'd recommend you first check your processes if something you're doing wrong. Coming over to the primary concern, Honeycomb Upsells and Upsellio app are two good recommendations. They tend to increase your conversion rates and help you stay on track along with providing you various other features.

Shopify apps such as ReConvert Upsell & Cross Sell and Justuno Pop Ups Bundle Upsell are the most trusted by other Shopify business owners. If you have had negative experiences with many upsell apps, consider the payment plan you were on with each app as they offer limited features for certain plans.

Upsell apps are useful to scale your business, and it is important to use the right ones. Some best upselling apps on Shopify are- ReConvert Upsell & Cross-Sell, Bold Upsell, Honeycomb Upsell and Cross-Sell, InCart Upsell, and Zoorix - Upsell & Cross-Sell. But you have already used one or more of these apps and haven't seen great results, then you might have to reconsider your approach.