Check this procedure: https://www.thevespiary.org/rhodium/Rhodium/chemistry/2ch.darzen.html (https://www.thevespiary.org/rhodium/Rhodium/chemistry/2ch.darzen.html)
It involves formation of the phenylacetaldoxime instead of the acetimine - I bet the yields will be much higher than with ammonia/NaBH4. Use acidic Al/Hg for the oxime. I believe all phenylacetaldehydes polymerizes on standing.