Sounds like a lot of fun work. If you can afford the extra wood or whatever materials you are using for borders, make multiple smaller beds. It looks nicer if you arrange them, and you wouldn't have to walk to the bed at all, and it will help control weeds. I would not put the sand in there unless you need to provide drainage. Depending on how much compost you put in, you're probably not going to have to worry about drainage. I would worry more about keeping it moist during the summer.

It is extremely inadvisable to import sand for a garden. If the native soil is very heavy add as much composted material as you can afford.

I'm guessing the purpose of the sand is simply yor add height to the soil inside of the beds. This is actually suggested in the square foot gardening books. But I really don't have an opinion on it. I understand why you're doing it though.

The one suggestion I would make is to make your bed not any wider than 3 feet. I am 5'9 (talk for a girl) and I still have a hard time reaching the middle if my 4' wide beds when the garden is in full swing.

As far as broccoli and cauliflower, cauliflower is a particular pita plant. At least in my experience. I stopped growing it. Out of probably the 20 different plants I've grown in two different years I only got one actual head of cauliflower. I've had better luck with broccoli but still maybe something like half produced heads. The rest either bolted or sat there literally from spring to first frost and just never did anything. But home grown brassicas are super tasty. They are heavy feeders so make sure you keep them well fertilized. If they never produce heads the greens are edible.

As far as when they are seedlings. I've never had a problem. Just keep the. Cool and don't let them dry out. Not much different than any other cool season crop.

None of then other crops you mention are worth starting indoors. Things like spinach and radishes of course are not going to transplant well. And beans and corn are such fast growers it makes more sense to just start them outside. Same with squashes and melons.

And yes, making a bed just for the asparagus is a great idea.

You're in Maine, I'm in CA!
You are listing things that are summer and fall/winter for us! LOL
Tomatoes, onions, peppers, corn, beans and taters seem to be mostly things to be planted in the spring (however, with our strange winter, we just harvested potatoes today!)
Lettuce, broccholi, cauliflower, peas, radish, carrots and spinach are all fall (cool weather) or possibly spring veges for you!
You might want to get in touch with your extension office for planting times in your area! Nancy

Is there something wrong with your native soil that you want to make raised beds instead of using the soil you have? From what you describe, it sounds like a huge expense of sand/compost/loam mix to fill such a big bed (not to mention the wood to build it with). And from what we usually hear on these forums, the stuff they sell for folks to fill raised beds with is often inferior to plain old native soil.

Is there a reason not to just mix some composted material (whatever you can find -- manure, etc.) in with your native soil?

I grow everything in native soil amended with as much organic matter as I can get my hands on, composted and then added either on the surface or dug in when the beds are new. As others have said, unless your soil is pure rock or has problems with toxicity, I wouldn't go to the expense of bringing in topsoil and definitely not sand. In the Maine you will want to add lime or wood ashes when you dig in amendments to raise the pH a bit since we tend to have acidic soil. Brought in topsoil will have the same issue, however, and will not necessarily be any better than what you already have.

Corn, peas, beans, radish, carrots, spinach, and potatoes are all typically direct seeded here. I presprout my pea seeds (soak 12-24 hours, drain, leave covered for 824 hours and then plant outside as soon as the soil is thawed and dry enough to work. Potatoes I chit (let sprout green for 2-3 weeks) and also plant outside under just a couple of inches of soil as soon as the soil is dry enough to work. I put soil around the stems just up to under the leaves as they grow. Radish and spinach also will sprout as soon as the soil is dry enough to work. Beans and corn need warm soil. Johnny's Select Seeds (Albion, ME) has charts of sprouting soil temperatures for different kinds of veggies you can check. I don't grow corn any more as the critters always harvest it just before I would.