If your tank is at 4300 and you are only filling to 3625 your tank can still do several more full fills. Also you do not have to fill to 250 bar. Any fill above the regulator setting will give you more shots. I am not trying to talk you out of a compressor, just trying to understand your 3 fill comments aligning with your 4300 psi comment. Different tank fills?
You would need to do more than one run or do something more than just fill a drywall bucket with water but a Yong Heng is an inexpensive tank filling compressor. I only fill a 45 minute firemans tank but it does a typical refill (3200 to 4500) in under half an hour. I use ice bottles in the water and have a couple small radiators on it. Another simple way to enhance cooling is to constantly replenish the cooling water. Mine is 4 years old and I rebuilt it with a new piston earlier this year. Cost about $20. YH compressors are crude but they work and are inexpensive.