arch/riscv/boot/dts/starfive/jh7100-starfive-visionfive-v1.dts also
uses YT8521. I added rx-internal-delay-ps and tx-internal-delay-ps
to rgmii_phy1 of mdio1 according to the prompts, and it now works
well using rgmii-id!

&mdio1 {
	rgmii_phy1: ethernet-phy@0 {
		compatible = "ethernet-phy-ieee802.3-c22";
		reg = <0x0>;
		rx-internal-delay-ps = <1500>;
		tx-internal-delay-ps = <1500>;
	};
};

&gmac1 {
	[...]
	phy-mode = "rgmii-id";
	[...]
	tx_delay = <0x0>;
	rx_delay = <0x0>;
	status = "okay";
	/* Motorcomm YT8521SC WAN port */
};

That looks O.K.

The YT8521SC seems to have issues with delays.
jh7100-starfive-visionfive-v1.dts says

/*
 * The board uses a Motorcomm YT8521 PHY supporting RGMII-ID, but requires
 * manual adjustment of the RX internal delay to work properly.  The default
 * RX delay provided by the driver (1.95ns) is too high, but applying a 50%
 * reduction seems to mitigate the issue.
 *
 * It is worth noting the adjustment is not necessary on BeagleV Starlight SBC,
 * which uses a Microchip PHY.  Hence, most likely the Motorcomm PHY is the one
 * responsible for the misbehaviour, not the GMAC.
 */

	Andrew