TD
I as turbodiesel
indirect??
It's a good sign that your bottom hose is substantially cooler. If concerned you could always get a new thermostat the wahler 87C or even the 80C wahler works good. IDK what climate you are on bc no location on your profile.
A level sensor would have been nice from factory but never happened.
If feeling slight overheating, temporarily increase the RPMs gently about 20% or to around 3000rpm temporarily since the waterpump is belt driven from the crankshaft so it obviously would process/flow more coolant. Also you could get the 5cyl diesel audi waterpump which has different gearing and spins faster.
Idk what else you could do..
Where do you live? At what temps did you feel-test your hoses?
And, YES, working temp gauge is absolutely critical though somewhat unreliable bc it shows overheat only when it is kind of late already. This temp gauge is your only feedback about the cooling system so it is HIGHLY suggested for you to fix it up asap if not functioning. Cheapest insurance to save a life.