Quick combination or text replacement for typing schwa
You can't achieve the first, as you'd have to have one instance for each word in which it occurs. As you've discovered, the text replacer needs a space before &amp; after.
The second idea could be done using Ukelele &amp; designing your own replacement keyboard layout. It's not an app I'm terribly experienced with, but I have managed to make my own modified structure without too much learning curve.
you end up with a 'new' layout copied from your existing one…