Forget Ookla or any other speedtest application. Are you getting full speeds while downloading games or torrents? If yes, there's nothing to worry about. Ookla speedtests are misleading most of the time. If you are getting good speeds(500-800 Mbps) where you actually need like downloading/uploading heavy files then there's nothing wrong with your connection. Steam and epic games servers utilize multi threaded download speeds.
Also, don't rely on ISP provided
ONT routers, they are not capable of handling a
gigabit connection due to weak CPU which is a huge bottle neck. Forget about gigabit, it can't even handle a 200 Mbps connection properly with 10+ devices connected. Remember that your ONT is not a router alone, it is a all in one device which handles PPPoE authentication, converting optical signals to Copper/Wireless, gigabit switch, handling wired/wireless clients, NAT etc. ISPs import it from china for very cheap in order to cut costs because of obvious reasons.
Put your ONT in bridge mode and use a high end WiFi 6 router like
Archer AX73 or
Asus AX5400 if you have a real use case of a gigabit connection and want to fully utilize it.
If you have opted for a gigabit connection just to perform ookla and other speedtests then you will obviously be disappointed. The only way you can fully utilize a residential gigabit connection is if you have a very internet heavy house with a lot of IoT devices and 25-30+ clients simultaneously streaming and downloading. Frankly, this is very rare and I still feel a gigabit connection is an overkill even for such a use case because most servers are very optimized nowadays and you can stream
netflix in 4K with just a 15 Mbps connection, for reference.
Netflix Bandwidth requirements. I am sure YT and other OTTs can be easily streamed in 4K with a similar speed.
Modern routers come with technologies such as OFDMA and client scheduling which further optimizes bandwidth utilisation.