The R2S will easily handle those requirements. It is more than adequate for your proposed use.
Make sure you order an R2S with the OEM machined aluminum case to pull the heat from it. It's worth the extra $7, which brings you to $32 plus shipping for an R2S. That's a good value for a gateway router with this capability. Beware reduced NAT throughput with its USB converted to second Ethernet port if you are bridging between the ports. That 's the only wart I can think to highlight on the R2S. Nonetheless, the R2S easily handles your 150 Mbps requirement. This review provides a good comparison of the R2S and R4S networking capabilities. This review provides good information on the R2S Wireguard and CAKE capabilities.
Used as you describe and with its metal case, it will not even come close to over heating. The temperature graph on FriendlyElec's R2S description is produced by loading the CPU 100% for 12 plus minutes. And it still handles it. And you won't be loading the CPU that high at 150 Mbps throughput, and it's unlikely you will sustain 150 Mbps continuously.
Budget is obviously a consideration for you, or you would not be looking at the R2S. However, I would seriously consider getting the NanoPi R5C instead if your budget supports it. For $56 plus shipping (metal case and 2 GB memory, plenty for a gateway router), the R5C gives you a lot more value beyond the R2S (double the CPU capability, double the RAM, on board eMMC flash, two USB3 ports, 2.5 Gbps ports and an HDMI port, which, OK, HDMI is kind of useless on a gateway router, but it's there). The R5C runs cooler too.
The R5C is supported in OpenWrt snapshot, which means it will be in future stable OpenWrt releases. Snapshot is pretty reliable - I wouldn't worry about not having a stable release for another 6-12 months.
I run snapshot OpenWrt on my NanoPi gateway, which has been an R2S, R5C and R4S (what can I say, it's a hobby) at different times. The R4S has the most CPU capability of those three and would be my pick if I needed to handle Gigabit CAKE SQM, but for CAKE SQM in the half Gig range with Wireguard, I'd pick the R5C all day long for best performance per dollar.
Edit:
The new R3S released since posting above makes the R2S completely superfluous. The R3S is only $3 more without eMMC, which the R2S lacks anyway and can in any case be added for another $5, and for that extra $3 the R3S has double the memory and CPU, and runs cooler. The R5C and R3S are now both supported in 24.10 stable.