I agree that a subscription for an extender would be money down the drain after a few months. A square Smart Hub 3 extender on EBay will set gou back about £60-70 by the look of it. Make sure you buy the right one. The older ones seem to be about £30 and I don't know about compatibility but it is in any case likely they are slower or perhaps less reliable if paired with the wrong hub version.
Maybe people have tried mix and matching BT hubs and BT extenders.
Another approach would be an ordinary tp-link or similar extender; I'm assuming nothing prevents them working outright. Or a long cat 6 cable!
PS I found BT very helpful once I got through. It tells you the likely max wait time which varies enormously. But check with your doctor before submitting to the neuron disrupting music on hold,
