I'm going to start with something other than your baby's sleep:
I try and sleep between DS going down & DH going to bed but struggle to sleep at 9pm...
Not being able to sleep when given the opportunity, or being unwilling to, when sleep deprived and exhausted is a Red Flag for postnatal depression. I mention this because while treatment for PND won't change your child's sleep, it will give you much more strength to cope with it as it is and/or deal with the challenge of improving it.
Food
Font worry about it for now. From 6-9 months the idea would be to maintain milk intact at pre-weaning levels, adding solids meals in addition to milk, not in place of. This then builds up baby's diet so that 9-12m you start reducing milk gradually as a means to increase portion sizes.
Bedtime
Your baby currently has such disrupted sleep that bedtime doesn't overly matter. Certainly not something to be worried about now. So if 9pm works for you, it's fine.
Cosleeping
Is there a reason you don't embrace this full time? That would be the easy answer to get everyone more sleep with fewer tears - the path of least resistance. Make the cot into a sidecar cot next to your (parental) bed and you and baby sleep there, cosleeping from the outset if needed. If DH can't deal with baby crying at night, he moves to the spare room not you.
This allows you to carry on feeding to sleep without having to cope with the tears of sleep training.
Cosleeping verses Independent Sleep
Will DS learning to sleep independently help? If yes how do I persuade a "fed to sleep" baby to sleep independently?
If you intend to be your baby's source of comfort (by feeding to sleep), then you need to also accept full time cosleeping as explained above.
If you intend baby to sleep independently in the cot, then baby needs to learn to go to sleep in there and not be fed to sleep. So to answer your questions: Yes & with a lot of either time or crying.
Independent Sleep
The basic premise to all sleep training methods is to teach baby to go from awake to asleep in the cot. There are a million different ways to achieve this, which is where the conflicting advice comes from, but they all have the same goal and end point.
The wonder of the dummy is that while actively sucking it is impossible to cry. So dummies help baby learn to sleep independently without much crying.
Your baby doesn't have a dummy. So the first thing to say is that your baby will cry. No it's, buts or maybe. Learning to go from fully awake to asleep in the cot will be very hard, distressing and cause a lot of tears for your baby, used to being fed/cuddled to sleep and without the intellectual ability to understand why. Don't start sleep training unless you're prepared to cope with the crying (back to PND at start of my post), because half-ass doing this will make it work.
Of all the millions of methods, basically the methods which involve less crying take a lot of time (many months) to make progress. The methods which involve the most crying will see progress much faster (days).
Your choice on sleep training method comes down to your own resilience - how much crying and distress from your baby can you cope with as they learn independent sleep? That needs to be balanced against how much longer you can continue with poor sleep.
Daytime Naps
Have all naps on something that moves, without you going out. It makes life a million times easier. Bouncy chair is ideal - Sit baby in it at your feet as you sit on sofa and foot bounce as you watch TV. Or pram pushed non-stop back and forth on the spot.
Reduce awake time. 3 naps per day isn't enough for your baby. 60-90 minutes awake time would be ideal, no more than 2h. Persistant and cronic over-tiredness will be making everything harder and you could do with a few days in focusing to get out of this downward spiral of less sleep making sleep worse.