Yours is a good application for a remote sensing power supply. In a nutshell remote sense power supplies regulate the voltage at the load instead of at the source - this is done to compensate for voltage drops in the cabling. Here is an example:
Remote sense power supplies may be purchased from numerous sources, but they are considerably more expensive than the "wall warts" used to power most RPis. They also tend to be higher-powered units that may not be suitable for RPi's relatively modest power consumption. For example, this unit made by Mean Well. Many "bench supplies" used in electronics labs have separate remote sense jacks or inputs to bring the voltage at the load back into the bench supply.
Note that the current carried over the remote sense wires is a wee fraction of that drawn by the load. Consequently, sense wires may be small gauge wires (larger AWG numbers) - they have virtually no voltage drop because the current they are required to carry is small.
There are, of course, DIY options to buying. There is nothing magic about remote sense - again, it simply takes its sensed voltage from where the load is located instead of where the power supply output is located. This blog on the Instructables website gives a fairly detailed account of hacking an older bench supply to use in remote sense mode. Similarly, this Electronics SE Q&A covers some considerations for doing the same thing to a different vintage power supply.
You may find a remote sense supply in that old desktop PC: Some ATX power supplies have been built with a +5 V sense wire (typically colored pink) connected to one of the red +5 V wires. However, as this was never part of any official ATX standard REFERENCE, a detailed inspection will be necessary to "qualify" its use as a general-purpose remote-sense 5V supply.
If you wish to power the RPi using its USB-C connector, you'll need a detailed wiring diagram for the USB-C cable. The USB-C cable itself may be a good choice for connecting a remote-sense power supply to the RPi as the shielded data lines should be good choices for carrying the sense voltage back to the PSU.