What i want to determine is until what point of it is OK merge those UXTOs in a TX
If you can make your payments without merging UTXOs, why you don't do this?
You care about privacy and try to merge UTXOs, is it something worth to consider for your privacy?
If possible, I will use each UTXO for different transaction, for different receiver and I will never to merge those UTXOs like you want to do.