Router replacement?

You could check if Merlin is still updating the firmware for that model and squeeze a bit more time out of it. ? 

As for Wifi coverage, my Router has no wifi at all, and I use 3 Wifi 6 PoE AP's for wifi coverage.  Anything that can be connected to ethernet is as well.

I'm of the opinion that a single router will not cover a whole house, but yes, that's very much dependant on the house size and construction.

If you move to EE you will find that they don't force you to to use their own router, although it does have very good WiFi range. Some others are the same. 

So depending on your personal requirements, you have two options. Get a new router now that will be compatible with Openreach fibre (most will be). Or do nothing now and just use the EE router when you move. 

If you have XGS-PON available, I believe GiffGaff will also let you use your own router from the ONT.  If you go VM, you will get the Hub 5X without modem mode.  EE with Openreach as noted, but they also still supply a TV recording box I believe, it that's of interest.  The world is your oyster, as they say  :-)

If it was me, I think I would try the EE hub first of all and see what it was like then get a third party router only if it was necessary.

I have a BT Smart Hub 2 (same stable as the EE Smart Hub Plus by the look of it, perhaps SH2 is the previous model).

My BT hub does not allow a guest network but it looks as if that has been added to the new EE hub.

My BT hub does not allow separate network names for 2.4GHz and 5GHz and it looks like that is the case for the EE hub (based on a few YT videos).

The BT hub is nothing special at all but it works reliably. I think the BT/EE hubs are capable of running EE's 'Hybrid connect' option where it will fail over to a mobile connection on EE (but needs more hardware to do this by the look of it).

Basic Wifi from the BT hub is OK. It was designed to work with BT wifi 'disks' to expand wifi coverage. Looks like EE has similar components to link together as part of a total EE system.

It looks as if you can buy all these BT/EE component parts off the shelf (rather than having to rent them) so, if you wanted to base your setup on EE kit, you could do that. Alternatively, as already stated above, get your own kit and plug a router into the ONT.

The biggest thing you would (hopefully) notice on a new fibre network is reliability. My BT FTTP connection hasn't missed a beat in nearly 4 years of use. Compare that with VM on 20 year old coax cable which routinely would reboot itself or go off for an hour or so or periodically go very slowly. It has been a night and day difference on BT FTTP.

My neighbour is on EE FF, using the provided router. Their 5GHz WiFi covers most of my house, which their Hub 5 never did.  As @goslow said, its settings are rather limited, but it has what most users would ever need.  They are very pleased with their recording TV box.

Can't comment on the EE CS, because neither of us has every needed to call them, the FF is entirely reliable.   VM need to catch up fast, if they want to keep their customers.