Yeah, I'n not surprised by the speeds to be honest. The WiFi is mainly the weakest link here.
Even though your Tenda Mesh is Triband both 5GHz bands are only 2 streams - so max PHY connection speed of 867Mbps - thoughput will probably only ever be ~650Mbps on a speedtest when right next to each other. As soon as you move them 5-10m apart I would expect the speed to drop. You may also find if you have them in a "row" so main talks to "2" then "2" talks to "3" then the throughput will drop/halve. You my not be able to see this in the settings as I'm not familiar with them
According to the Tenda app it looks like all the nodes are talking to the all the other nodes at the same time; there is no obvious "node 3 has to go through node 2 to get to the master node"
The devices only geting 85Mbps - hmm, are they on 2.4GHz which is slower - or really old? I'm guessing 2.4GHz
Very weird - just retested with my phone which is definitely capable of 5GHz and now speedtest.net says 550Mbps when sitting right on top of the master node and 225Mbps when sitting on top of the
furthest node; which is a heck of a lot better

. I think the Tenda nodes are not very quick/smart in handing over control to over nodes so that probably doesn't help the tests.
Agree the C7 is a bit old and may not be able to route any faster on the WAN, especially if PPPoE is required for Hey Broadband.
Is the C7 doing anything other than connecting the ONT to Tenda MW12? It looks like the Tenda can do routing, either DHCP, PPPoE - have you tried that to see if you can get the wired WAN speed a bit quicker?
AFAIK PPoE is required by Hey!Broadband
So, currently, the C7 is just (a) connecting the ONT to the MW12 (b) the DHCP controller (managing a very small reserved IP list) and (c) acting as a switch for a couple of, nearby, hard wired devices. Long term, it will also be in charge of the DNS settings for all the devices on the LAN. I am aware that the Tenda can do PPoE and DHCP (not sure about reserved IPs) but I don't think it can do custom DNS settings. It also only has 1 WAN port and 2 LAN ports on it which may be a concern at a later date.
Unfortunately the only real options are to spend a few hundred quid on faster AX kit or wire the nodes. Even then as you may struggle to achieve 900Mbps on a slave node at longer distances.
Yeah, think I might get some Deco DX20s from Amazon - and just return if they don't improve things much.
If I was going to wire the nodes I would also run wire all the way to the wired devices (which are currently just wired to the furthest node) but ... it is certainly the last resort at the moment