This is me in 2019 saying don’t buy a CAT18 modem yet (when they were just released) and ignore headline speeds. LTE Categories 12 vs 18 or higher - #2 by MartinLangmaid
Today the difference between cat18 (LTE-A) and cat 20 (5G) is a similar story. You’re not buying it for headline download speeds of 2Gbps on a 5G modem. You’re buying it because it can do more aggregated carriers and support more concurrent spatial LTE streams on more bands so you get better reliability and can connect to more towers in more places.
But just like back then I’d still say that the Transit Duo (dual CAT12) or CAT18 Transit (single modem) is better value and more balanced than a Transit with a 5G modem.