The yield curve reflects market participants best estimates about future yield plus a term premium, it cannot be used to estimate the future expected return of bonds. If you want to know the future expected return of bonds you need to know what the term premium is net of expected interest rate changes, but this cannot be observed from the yield curve.
There are two conflicting arguments that can be used to pick the best combination of bond durations:
The diversification argument. If you are diversified across the yield curve, that results in lower risk.
The bet-against-beta argument. Frazzini et all
has shown that the sharpe ratio of bonds decreases as the duration increases. According to this argument, you should prefer lower duration bonds.
If you ask me, the correct answer is to stop wasting your time with micro-optimizing your bond allocation. Just buy total bond market and focus on the things that actually matter (in this order): stock/bond ratio and lifecycle investing models, international diversification with equities, understanding annuities, factor investing, international diversification with bonds.