Hard to evaluate weight without a photo at least.
@paintedpennypony have you worked through the Henneke charts? What are the fat pads like at withers and tail? Can you see ribs?
Horses can have saggy bellies from gas from poor quality hay or former pregnancies or lordosis without being overweight. That’s why belly isn’t included in the Henneke charts.
Wild horses will enter fall obese and can lose a lot of weight over the winter. This helps keep them from getting metabolic syndrome. But it isn’t desirable for a riding horse to be 7/10 in September and 3/10 in March!
Domestic horses should stay around 4.5 to 5/10 year around. But they tend to creep up in weight because they have lots of grazing in summer then no work and lots of hay in winter.
Whether obesity will have immediate consequences is very dependent on the horse. I’ve certainly seen people tip their horses into metabolic syndrome and laminitis just on overfeeding hay. Also it’s worth testing for Cushings if weight is out of line for the amount of feed (either thin or fat).
Once a horse is metabolically compromised it’s a lifetime of soaking hay and limited grazing.