Where do collectors hold cash between purchases — OKX VIP fees + OTC desk review

Garage of three air-cooled + a daily. Like most of you, cash flow is lumpy — sale comes in, next car sometimes 6 months out. Started using OKX as a holding venue (https://okx.com/join/CODE777, code CODE777 on signup = 50% off fees). What works for collector-tier money: VIP fee schedule, OTC desk for 6-figure fills with no slippage, cold-vault custody with published Proof-of-Reserves. Tracking record is clean. Anyon