If you have over 70 devices connected, you are moving out of the realms of a home router into professional, enterprise equipment… The limit for most radios is 30-40 devices, some enterprise kit has multiple radios, so can cope with 80 devices or more. Bandwidth is also a problem, if they are all trying to talk at the same time, the airwaves will be choked, although a lot of IoT is low bandwidth and it just sends keep-alive packets, until it is needed.
I use Unifi enterprise kit at home, it is reasonably priced, but you need a Unifi controller to manage the wi-fi and you need to understand wireless networking and networking in general (VLANs, for example) to set it up cleanly, but it is the simplest of the enterprise tools I’ve worked with to set up - we have Dell, Zyxel (enterprise), Cisco, Aruba and a few others at work and Unifi is by far the easiest to manage. Unifi’s controller is also free to install on a Linux or Windows PC running as a server.
At home, I have a Unifi Cloud Gateway behind my router and 2 access points connected to that to cover our flat, with the router in passthru mode.
If you need home kit, it is more difficult, but something like Eero might be a way to go, they can allegedly support up to 128 devices.