OK, you asked 'what would you do' rather than 'do you know what you are doing'....
I would consider two options. 1, do the minimum and let it retrench gradually. 2, encourage the retrenchment. The reason I say this is because the area I've circled in green looks to have good internal growth, suggesting natural retrenchment. To encourage this I would cut back reasonably hard, but not really pollard it as I suspect the shock would kill it.
For aesthetic reasons (since this seems to be a consideration here) I would use drop-crotch cuts where possible, working on the basis that although this makes them relatively heavy, there is more work to be done later once the retrenchment growth takes over, so although ash isn't naturally durable and hence is likely to rot before it heals on heavy cuts, the remaining heavy wood above the cut will be taken out before it becomes unstable. Where there is no suitable point to drop-crotch to, such as the branches overhanging the road, I would cut back as shown and then coronet up the end, particularly on the underside, to simulate a slight rip fracture.
One or two of the central cuts I've shown higher up are slight guesswork as I would want to walk round the tree to place them.
My long-term aim would be to end up with a natural looking retrenchment/pollard, with the majority of branches springing from the height of the horizontal branch over the road (which is why I would shorten it to encourage growth near the centre, rather than remove it), but maybe leaving the straight section above alone too until it gradually disappeared by itself. May well become hollow, but with the weight off it I would expect it to be stable, even if future work was erratic, although it would require the introduction of a maintenance cycle, and it would be great veteran habitat.
Please do not take the above as advice - I am not qualified to give it! It would work nicely on an apple or pear though and I would get away with it on a plum....
I would, however, be very interested in David Humphries' view of just how wrong I am.
Alec