A single RB2011UiAS-2HnD-IN should easily be able to handle that workload. Even a hAP ac² would be able to handle the network traffic, the wireless will depend, the RB2011 will probably be able to handle it, the hAP is a bit light in the pants on the wifi end (most Mikrotik devices are) you can also possibly look at gettings a Hex S for routing and a proper AP from a different brand for Wifi.
I'm thinking about RB450Gx4
with quad-core CPU and 1GB of RAM + a few cAPs for Wi-Fi, but the fact that it has no USB-port is a factor as well.
What will you do that concerns you about the memory.
Hap ac2 has 4 cpu and that memory is more than sufficient
I have no doubt that for now
a far superior hAP ac2 will do work much better than my old and rusty 7-years old router that also has 128MB RAM. Question is how future-proof it would be in a couple of years, if modern day mid-range consumer routers (120$-200$ already have 256MB - 512MB RAM and comparable dual/quad-core CPUs.
Memory is mostly for buffering when network contention occurs.
I intend to try out ad-blocking scripts, fine-tune QoS rules and perhaps even a load balancer (might get a second provider with the cheapest subscription plan or a 4g internet, this is why USB-port is highly desirable).
How fast is your internet connection?
For now it's 100Mb\s, but I intend to switch to 1Gb\s after I finally buy a new router.
How much ethernet ports do you need?
5-port router would suffice for all my needs. I have a couple of gigabit switches anyways for a TV-box and X-Box console
For WiFi, is it a big house with multiple stairs? Concrete?
Do you need high bandwidth between VLANs and security?
It's a 2-story bricks house, and my old router's signal wasn't powerful enough to cover all it's area, so I bought a cheap hot-spot that covered the rest of the territory and a backyard. I think I will buy a 1 or 2 of cAPs eventually, but for now my budget is somewhat limited by ~150$ that I can spend on a router (be it wireless or not).
I don't need a really high bandwidth for wireless devices, on a contrary I limited it because my old router struggled hard for the last year.
The hAP AC2 is able to push about 2 Gbps in routing, plenty of horsepower to handle a regular internet access. If your inside network is all bridged, then you use the internal non-blocking switch. If you have a large house with many floors, then you could start and see with the hAP AC2 and add a wAP (handled by CAPsMAN in the hAP AC2) if signal is not strong enough. On top that, adding another AP will increase total available bandwidth providing client nodes are dispersed and will connect to the nearest AP.
Personnaly, Mikrotik wireless done right will perform well. We run 20 APs that cover a whole manufacturing plan and we ended up with at least 50 Mbps everywhere in very harsh conditions with a mix of hAP AC (offices) and SXT (inside the huge metal multi-floor manufacturing facility). CAPsMAN is running in a CRS112 and it is easily able to handle all the traffic, internet access is 400/50 Mbps.
I'm mostly concerned about an ability of hAP ac2 to handle high workloads in a couple of years when majority of mainstream consumer routers might have ~256MB - 512MR RAM and quad-core CPUs (some actually have similar specs already)