Agree with everything said so far about periodic tenancies but can understand that you would feel more secure with a fixed term.
I know you said they have been sadly lacking so far, but you could contact the landlord directly and explain your concerns and preferences and see what he has to say.
As someone said, he may not necessarily be aware if the agent is answering on his behalf.
Sounds a proper mountain out of a mole hill.things go wrong and especially during covid take age to get sorted, if the problem is fixed and you like the place, stop moaning and live your lives.
Can only agree with previous replies. Boilers do break down. It is not the landlords fault, as long as they fix or replace them in a timely manner.
Try and speak to your landlord directly with your concerns. Periodic is quite normal and probably to save having to pay for renewals through an agent.