A Netcologne technician punched down my connection to the ZTE box today and things worked out about half as good as I wished. The 190-T immediately synched up but for some reason the connection is capped at 500/100 instead of 1000/100.
My ongoing theory is that although the ZTE box is capably of reaching 1Gbit/s on a 106Mhz bandwith modem, a wrong profile prevents a successful sync.
Sadly the 190-T (and all other MT-5321 based modems) only supports 106Mhz profiles, Netcologne is usually using 212Mhz profiles though, as the Fritz!Box they offer as a default is capable of both.
Next steps would be a call with a Netcologne technician on monday to hopefully figure this out and reach full speed with a new profile/some obscure setting on the ZTE box or I might have to buy some (at least okay-ish) external modem from Draytek and eat the cost of the 190-T.
Update: Netcologne will send another technician qualified to do diagnostic work to sort this out. According to them it should just work ™️