Welcome to ObjectScript and the Community.
As you are still learning ObjectScript I hope you don't mind a small amount of extra guidance on what you are doing.
In general abstracting ObjectScript string operators and built in string functions with Macro's is going to produce a non standard solution. Whilst you will be an expert in Reinhard String Functions, you won't be a fully grounded ObjectScript developer and the code you produce will be less readable and maintainable to those that are.
Learning native string operators and built in functions is an important step in becoming a competent ObjectScript developer and its really worth the investment to build these important foundations in your new language skillset. In practice there are very few operators and string functions to learn and the required investment will work out less than developing and unit testing your own library. That's not including the time to write bespoke documentation that other developers would then need.
I can also understand why auto complete goodness might also look like a useful asset in this process, but as soon as you are fluent in ObjectScript foundations, the code will just be streaming off your fingertips, and constant pop up suggestions would just be a distraction.
A few more things to consider. Overuse of Macro's can lead to a condition I call Macro Soup. The more Macro's that are used, the less readable code will become. If you start including very common functions such as strings then you will soon hit this problem. It's great to see you are already exploring these powerful features of ObjectScript, but IMHO its best to be non liberal in using them.
You might also be interested to know that InStr() is actually supported in Caché, both in its SQL dialect and with the Caché Basic transcompiler. If for instance you are a Visual Basic developer shop and will continue long term recruitment from this skills pool, then Caché Basic might make more sense to you. That said, you will never be fully rounded without also being able to read compiled ObjectScript as well.
One last recommendation I always give when teaching ObjectScript is to write your own cheat-sheet. The process itself tends to solidify knowledge more quickly and having the cheat-sheet on hand will help to jog your memory.
Have fun learning!