You have two clips joined together - either just by a metal loop or make a short nylon strap if you prefer. Then you clip one to your belt (which stays there for the duration of the walk) and you clip the second one onto the lead at whichever length - the very end if you want him to have the whole line, or to one that only gives him a few feet of lead (so like a regular short lead) for road walking, and you could add more if you needed more options. No clips on the lead itself adding to the weight, only a couple/few D-rings. I'll try to doodle it out if you're still not sure!

I like the idea of the clip on handle, but I think that will be too much of a faff to get it on and off.

I've had a bit of a play again and I think I've done how JesterJenn suggested and made the handle flat.
It seems to be completely flat when used as a long line without velcro, so less likely to get caught on things, and then can be opened out easily to a handle. Possibly not as strong as option 1's design but I feel happier that it's less likely to get caught on things

I do mine similar to some of the other suggestions, I put a trigger hook on each end & the attach D rings along the length, first one at handle length & then another at standard length so you can make it into a 40" standard length, then another further down. You can attach them by using a small piece of webbing same colour, wrap around the d ring then sew it on to the length as a square with a cross through for reinforced stitching.

I had a play again last night and pinned lots of things together walking Cooper around the living room with it,lol, but finally settled on this design.
It seems to make the handle completely flat when there's a little bit of pressure on it so nothing will get caught hopefully, and it's very easy to get to the handle and hold it then let it go again when we are out and about without having to faff too much

That's almost identical to how mine look, and they are really strong to be fair.

I'm glad they seem strong, I was a little bit worried but it seems quite firm when I tug on it, and I used the extra strong stitch pattern.
I won't be using the handle much but at least it's there is I need it, and it saves me having to take another lead when he will be on the belt.