I see the backing process as a continuum from birth through all their handling and learning up until they're riding away confidently and happily, at whatever age that is finally achieved!
Before beginning work, with any baby I would want body handling to be completely established, ie they're happy to have you touch everywhere with anything, be it your hands or a rope or a lunge whip. Then I would also want them to be completely happy with halter / lead work. So completely clear in their minds about giving to pressure and stopping starting and following/turning in-hand politely and calmly. Ideally these things are established as they grow up from foal to yearling and 2/3 year old, and with Daisy you've had her from early enough that you will have these things already too. Obviously many youngsters are just left almost feral until someone buys them aged three, so they may take a little longer to get the ground basics covered confidently.
I try to aim to have a three year old obedient and calm and happy walking and trotting in-hand on a long rope, listening to vocal commands for forward and slow down, stand and back up, moving over sideways and yielding hind quarters and forehand from a light touch in-hand. Following this, I move on to more advanced commands like basic lateral work in-hand, moving over and opening the shoulder, giving to light pressure aids, crossing the legs and learning to take their balance back over the hindquarters a little. All this is still in a halter or cavesson. I would also want them to be confident, happy and calm away from home, both at exciting places like shows and just walking out on mini in-hand hacks seeing the world and meeting traffic/wildlife/other riders etc etc.
As for introducing equipment, given the opportunity I do this from birth onwards too. Then it no big deal about what comes out of the tack room and where it ends up being put

Both Tonka and Orio have been with me from birth and both have had the same treatment. Both have had numnahs plonked on them from very early, had things round their bellies, on their heads, under their tails, round their legs, all over. Orio used to "wear" my saddle on top of his rug aged 6-9 months while I shut the gate into the field as he used to meet me at the gate on arrival and walk with me to the stables

As a result, he is utterly calm and confident with any equipment on and around him, and I would expect no issues at all when he comes to be saddled later. Tonks was the same, backing him was "just another day" in his viewpoint!
Next stage for me in the learning curve for them is learning all the same stuff they did in-hand, but at a distance. So on the lunge or loose. After that, I introduce a bit on a light headstall and repeat all the in-hand and lunge/loose work (but without actually touching the bit yet). Finally, I start in-hand work with the bit. Introducing contact, encouraging the horse to mouth the bit and feel it and get an idea of how to hold it and respond to it. Then I repeat all the in-hand work above but introducing the reins as an aiding tool and converting the movements from voice and hand pressure to reins/bit and hand pressure.
Once they've got the hang of working in-hand and on the lunger going forwards, sideways, stopping etc with a light rein acceptance I would consider chucking a body over the top and introducing weight on their backs. Then progressing to walking forwards (lead at first, then lunged) and stopping under the weight, running through all their familiar exercises from their ground handler, but with a rider weight on board. Then once they're happy with the weight and its visual, physical and emotional impact on them and their movement and balance (which usually only takes a couple of ten minute sessions if they're well prepped), I start letting the rider apply the requests with light rein contact and leg, transferring the control over from ground handler to rider. I find if they've done the in-hand work enough, they already understand leg aids and are happy to accept the rider controlling them pretty quickly.
From there I usually then leave the school work alone for a bit and hack out gently, building fitness and strength and confidence with their rider before I ask anything more demanding in the school. I'm a big fan of turning youngsters away for a holiday after they're backed and hacking out nicely. How long for depends on the individual/time of year/facilities etc but I think it gives them time to think things through and they come back more mature and stronger and ready to go on.
I won't back Orio until he's four. He's still growing like a weed, and is still so lanky and leggy that I don't think he'll fill out and be mature enough until he's four

but he's been shown in-hand and does basic in-hand work with enthusiasm and obedience. He has worn a bit (twice

) but with no contact on it, walks out in the Forest, deals well with other horses and riders, wild ponies, cows, pigs, donkeys, water crossings, hills, gates, open moorland, traffic, travels well in his trailer and it is always commented on at shows how confident and polite he is

He has a very active mind and enjoys playtime in the school with me. We usually work loose or in a head collar and he is incredibly clever, so keen to please, he's adorable! I do stuff with him probably once a fortnight. He's always keen to be caught and is gutted if I take one of the others and not him out of the field, so I take that as a sign that he doesn't feel overloaded!
Tonka was very different. He is clever but only on his terms

clicker has worked wonders with him though, greedy sod

He wasn't backed until he was almost 6, so was a very late starter. I sat on him at four and he felt drunk. He is a
BIG horse and he just hadn't got the hang of his own body yet at that stage. I probably could have got back on at 5, but I destroyed all the ligaments in one knee and had a lot of time off riding, so he got another year off

By six he was totally confident in his own body and could balance himself properly. He was 100% better in-hand and on the lunge and when I got on he was fine. He hasn't looked back once

and I'm glad I waited for him to be properly ready. I still do a lot of in-hand work with him, and I find it really helps to teach him new things from the ground first before attempting them under saddle, as well as giving me the opportunity to watch him move and keep an eye on how his balance and engagement is progressing. I don't think that the ground work should stop just because you've got on board. It is useful and helpful as part of their ongoing learning throughout their ridden careers. I still work Noos in-hand and she's 17!
Gosh, that turned into a bit of a ramble, sorry