This is not a simple question to answer. The Eero is designed to be part of a Mesh system, requiring you to purchase extra nodes to get the best benefit from it. The Sagemcom might require WiFi extenders of extra APs (wireless access points) to get similar coverage within your property. The Sagemcom is as you say, far more configurable than the Eero.
WiFi 6 is the latest standard on offer from TalkTalk, but to get the best from it you need to have devices that are capable of WiFi 6 also. However, it is not the latest standard, that is WiFi 6E, which is a game-changer, as it introduces a third WiFi band - the 6GHz one. I doubt if any WiFi 6 access point or adapter can be upgraded to WiFi 6E via a firmware update. I would guess that it needs new hardware.
WiFi 6 can be faster than WiFi 5, but you need to very carefully consider the AC & AX classes that both the router/AP & devices support. AC classes go up to the AC5300, but the Sagemcom supports AC2200 which gives maximum theoretical speeds of up to 1.733Gbps on the 5GHz band & 450Mbps on the 2.4GHz band. The AX classes of WiFi 6 start at AX1800 which offers only 1.2Gbps on the 5GHz band, but offers 574Mbps on the 2.4GHz band.
However, with both the AC & AX classes, both the router/AP & the adapter must both be able to support the above classes to stand any chance of reaching these speeds.
However, speed tests will be limited by the sync speed achieved by the ONT modem.
Bottom line is, it is down to the adapters you have, the environmental factors affecting your WiFi signals in your property & your choice. There cannot be a definitive answer I am afraid.
