What’s the fairest and most practical solution for managing drink costs on boys weekend away where people drink different amounts?

Your trying to mix alcohol with a sense of fairness. Great intention but doomed to failure. If the people involved dont socialise often enough for any imbalance to even out over time, or for them to just not care then its best to just let people buddy up or look after themselves. If you try to organise any kind of system that involves people tracking who does what and when or start logging info into apps that split costs theres a high chance of argueing by the end of the night Its easier for everyone if theres smaller groups to manage themselves especially if any of them have less "disposable" income which is never an easy discussion. (Shouldnt be but ego is what it is) I stopped drinking 20 years ago and when im out i have no issue being the odd one out who just buys his own drinks and leaves everyone else to figure out what they want to do.