what digital cables for my dac?

First of all, don't use a half meter. I had a very expensive Wireworld Gold Starlight III+ half metre cable with my first separates, but the bargain-priced Apogee Wyde Eye in a 1.5 meter length placed the instruments in the soundstage more clearly. The WW cable was superb in many ways, but length matters. If you're not sure you believe this, there are a good many posts on the subject archived here.

As for a good digital cable at your price, I would suggest the Ven Haus cryo Pulsar. It is almost as good at timbres, warmth and detail as my old GS III+, better at dynamics, and much, much cheaper. Another possibility might be either of the Atlas digital cables available from UHF magazine. They ditched their Gold Starlight in favour of Atlas Opus--again, much cheaper.

VH Pulsar

UHF cables

Whatever you get, though, make sure it's a 1.5 meter length.

Although the starting point using a "cheap Sony player" seems to be your weakest point, the upgrading of digital IC will let you know when you have heard all there is to hear from your transport, in the digital domain.
I can recommend using matching Mapleshade digital IC's (on either side of your Monarchy DIP), inexpensively excellent without being too revealing of transport inadequacies. Also, can point you toward a subsequent upgrade to match a more capable transport in your future: Auricle Audio Design Encore Signature digital IC.
Audiogon member Joemazzaglia sells these custom crafted digital IC's which truly excel at revealing everything your digital components can do, which is much more than you expect, if accustomed to most other IC choices. Priced too low to elicit budgetary shock and awe, which is a good thing.

I'll second the VH Audio Pulsar.