I’m planning to make a modular synth from scratch, but I need to start with the PSU. Do you see any issues with this schematic?

The main difference between this design and traditional linear PSUs is the replacement of the transformer/rectifier/filter circuit by Mean Well IRM AC/DC converters. The linear regulation circuit is basically the reference design for the 78xx/79xx.

Do you think there would be an issue once scaled? The AC/DC converters have a lot of headroom, as I plan to make up to 3 regulation circuits like so:

full schematics

(the regulated outputs will actually never go nowhere near 1A per rail)