There are all kind of strange restrictions on which cable modem work. It can vary a lot even within the same city because of difference in the equipment the ISP is using.
In general you must look at a cable modem as media converter. Its only purpose is to convert the signal on the coax cable to signals on ethernet cable. As long as it meets the requirements for the plan you buy from the ISP it will function fine. In this case some of the modem you list can support 2.5gbit.
If the ISP does not actually require these units for the 600mbps plan then you are paying for extra features you will never be able to use. You really want to buy the lowest cost product that is on the ISP list for the plan you purchase.
Most routers will not bottleneck the connection. If it is really old it might only have 100mbps ethernet ports. In general most devices, even very cheap ones, can run close to gigabit speed WAN/LAN. Now wifi is a completely different story. You likely will get nowhere near 600mbps and it will be much much less if you are
using "mesh" repeater systems. You really want to avoid using any form of wifi repeater if there is any possible way.
My guess is you either have something with a 100mbps port on it or you have a bad ethernet cable. Most cable modem even fairly old ones can run well above 100mbps. BUT there maybe artificial restrictions based on the cable company implementation in the area you live so you are back to first checking the compatibility list on the ISP website. Nobody on the internet can really tell you unless you want to post your address and zip code