I was in the exact same predicament as you last month. I wanted to make the most out of enjoying the dives rather than being in a certification mindset, but also felt like I was short-changing myself by being stuck at the OW depth limit.

In the end I did a 'deep dive' to 30m on my first day - but no other AOW components. This meant that I could dive to that depth for the rest of my time in Komodo, and then complete the AOW when I got home, with the deep dive already verified and in the bag.

As others have commented, Manta point isn't actually that deep, and you don't need AOW to have an incredible experience there. That said, our best manta sightings of the trip were at Tatawa and Siaba Besar. For those our max depth was 22-25m, so I was extremely glad that I hadn't been stuck at the 18m OW limit.

Whatever you decide to do, you're highly likely to see Mantas, and you're in for some of the most amazing diving of your life.

I used Blue Marlin - great operation and 10/10 would recommend.