A brief summary of your quest would be more easily split into two parts, with sub-parts for the two sections.

Section one is the tough one. Designing a custom part of the type you describe is going to take some CAD skill. If you had substantial experience, aka skill, you would be able to save a bundle on the out-sourcing part of your question. It's good that you're willing to learn the process, because that will benefit you in the long run.

I'd recommend Autodesk Fusion 360, free for hobbyists and low grossing businesses, requires renewal each year, no big huhu. It will give you greater control of your model revisions, especially if you learn parametric modeling aspect of the program. If you're more of a coding/programming person, OpenSCAD (also free) is a descriptive language modeling program that also provides great parametric control.

On the printing side, ABS is not a pain, or at least isn't if approached with an open mind. I've been printing custom brackets these past few days in ABS and have had a couple failures, but I expect to have the occasional problem. Build your model in the CAD program of choice, print it in PLA and when you're happy with the iteration, print it yourself in ABS or farm it out. Once you have a good design, you aren't limited to PLA or ABS or PETG, because you can use an outside print resource and get exotic materials like SLS nylon, strong, heat resistant, attractive (no layer lines!)

The model building is the right place to start.