I empathise, FW, but am of mixed feelings.
I think it’s about age and stage at least as much as size. If we had tiny rings for our tiny sheep, we’d have no issue doing it by 7 days, would we. Their testicles drop by about day 3, if not present straight away - or at least, most of my primitive boys’ have. So if the limit has been set with reference to the feelings - physiological and emotional - of the lamb, then it is less than ideal that we can’t ring them within the 7 days.
I’ve given it a lot of thought over the years. I’ve wondered whether maybe the Combined Flock Book, the Rare Breed Survival Trust, and the relevant breed societies could get together and organise the manufacture of tiny rings and tiny elastrators. But no doubt the setup costs would be huge, and the market extremely limited.
With my flock in Cumbria, I mostly found that if I couldn’t do them by day 3, I wasn’t going to catch them easily, so would gather the new ewes and lambs up once or twice a week and do anyone not yet done, if possible. Some would escape the gather and then be too big, or one or two might have one testicle not fully descended the first time, and be a little too big the next time. So we bought sheep burdizzos and did anyone I’d missed when they came up to the farm - but depending on the year, they might be around 3 months old at that point, and the testicles would be a fair size. One year we asked the vet to castrate the 4 I hadn’t ringed, and they did, but said they’d rather not do them at older than one month in future. Well if we were bringing them up to the farm at one month, we could burdizzo ourselves, so that’s what we did thereafter.
Now I’m without an experienced burdizzo operator, so I’m back to doing it with rings. All the crosses are fine, I can pretty much always do them by day 7. And the fields are smaller here, so I can get them caught

.
This year I used a Shetland again on my new entrants, both of whom are crosses, and no problems. But last year I’d used a Heb tup on my first timers (both crosses again, and neither as small as a Heb) and one other sheep, and the tup lambs were a nightmare. Tiny testicles, slow to drop, if you thought you’d got them and they slipped back, the scrotum was so woolly it was really difficult to remove the band.... I said never again. If Hebs were my breed, I’d have to decide, as you have, to ring later than the guidance, or else to get someone else trained with the burdizzo.