what is the current value of the tilt ? (mechanical + electrical). And how far is the area you want to cover ?
How high is the site compared to this area to cover ?
After you give me these values, maybe it will appear that your tilt is good enough. In this situation, you might want to :
add one more antenna in order to do RX DIVERSITY
add two more antennas to do 4 Rx Diversity and 2 Tx diversity
change your antenna model (use one with more gain, more directivity)
add a tower mounted amplifier (a.k.a TMA or LNA)
Since the main problem is uplink, I can’t ask you to use a high power TRX : this would increase onl the DL strength, not the UL.