If the situation is as you've described, i.e. routing will need to pass through your router, as almost all traffic is not local (excluding some/few hosts accessing one internal server for "accounts purposes" [low volume?]), you could use your router, on-a-stick, for all routing.
That noted, most L3 switches often have much, much (more) more capacity for routing than most "comparable" routers. I.e. if you're doing any local routing, beyond low volume, a L3 switch is a better choice.
However, many times I've used both L3 switches with routers. I.e. L3 switch routing in conjunction with the router. (The router being used for features not usually available on many switches, such as NAT/PAT, advanced QoS, non-Ethernet media, etc.) The goal being to perform all routing such that's "optimal" for our goals. In such combined routing environments, you might have all traffic routed through the L3 switch, with the L3 switch switch routing to/from the router for non-local site traffic.
Using both a L3 switch and router (the latter being basically a router-on-stick), together, it's also possible to have both L3 switch and router having interfaces in all local networks, using L3 SVIs interfaces as the gateway IP for local networks. This allows traffic returning from off-site to transit the L3 switch at L2. Optionally, if you allow the L3 switch to "redirect", it can also inform hosts to redirect off-site traffic to the router's IP, again bypassing the L3 switch's SVI. (NB: generally, as most L3 switches perform routing at the same speed/capacity as L2, there's often little advantage bypassing the switch's SVI for off-site traffic.
(If the prior is unclear, please let me know, as sometimes learning to use L3 switches, as both L2 and L3, concurrently, can be a bit confusing.)
