If as the manager, you don't know enough about the codebase to get a sense of "last X development efforts on component Y took about Z weeks of effort", then it is your job to get to know the components better.
Note that doesn't mean building up the expertise to be able to do the changes yourself. Just that it should not be completely greek to you.
I've never worked on a project where I was estimating something at a scale of 6 months. But if you ask me whether something will take 1 week or 4 weeks, before I've broken the task down, my intuition is going to scream "I don't know the answer to that."
If I told you "Somewhere around 1 week to 2.5 months", would you accept that answer? Or would you think I was trolling you and we should have a conversation about my performance and place at the company?
If I instead told you "2 weeks", how would that be anything other than a lie?
That said, my main point is that time estimates lead to bad negotiations. If someone says it will take six months and you need it in five, what is on the negotiation table? Just a month? This is how our industry often finds itself in crunch time, making up for time estimates gone awry.
Whereas if there is a list of things that can be negotiated, you can order the construction such that things are natural cuts.
I agree no estimation process is perfect. Nor do I think you should do comprehensive estimates on new work. However, the thing you want to know it's how much work there is. Not necessarily when you'd like to release a year from now.
Odds are high you have a deadline. So the incentive is high to keep the estimate below that line.
If you know of anything to read, watch, or work through to learn to estimate tasks, I would be extremely grateful if you could link or describe it here.
As it stands, I’ve only ever been able to estimate tasks if I’ve done similar ones a few times before. If asked about a new type of task or one with a new toolset, I would currently have to refuse an estimate more fine-grained than a week—-the stress and shame of lying to you would be too much otherwise