"Mid-price" can cover a multitude of values.....You might want to consider the Asus routers that support Wifi 6 and the company's AiMesh. Wifi 6 gives you the latest and greatest wifi for devices that can support that, and AiMesh would enable you to add another Asus router to create a true mesh system if you find a single router still doesn't give you the coverage you need. You're talking about £90-160 for one of those.
Be aware that the Hub 4 has a speed potential of 1.17 Gbps, but the ethernet ports are only 1 Gbps, so you're going to lose 15% or so of max speed potential in modem mode. Probably doesn't matter compared to the benefits of a good third party router, but worth knowing. Also, there's a modest range of oddities reported of third party routers not getting full 1 Gbps speed from a Hub 4, even though the hub is getting the full speed to VM's network. I've never seen a good explanation why that might be, and whether every case has been resolved, but make sure you check out any new router performance fully within the supplier's no-fault returns window.
As for heat, offloading the router processing won't make much odds - the heat generation is caused by the incredibly intensive mathematical processing required to keep 33 DOCSIS channels alive, which is largely the same whether you're using the connection or not, although in most cases you won't notice either the fan or heat problems (well, not for a few years until the heat sink and grilles "fluff up").
