If they were mine and I didn't want to bin them I'd soak them in water and insert some tight fitting shoe trees with the laces done up tight. They'll shrink as they dry and the deep creases will become shallow. Then put leather filler in the cracks. Quite a tricky job. Has to be done with many thin layers. You tint the filler first so it matches the leather. Then sand it. Put the shoe trees in the boots every night so the shallow creases don't become deep again. It's quite a big ask to expect the filler to stay in. Best to replace the boots with lightly used ones from ebay. Prices are often v. v. low. If you're wary of fungi from the previous owner you can kill them with UV light.
If they were mine and I didn't want to bin them I'd soak them in water and insert some tight fitting shoe trees with the laces done up tight. They'll shrink as they dry and the deep creases will become shallow. Then put leather filler in the cracks. Quite a tricky job. Has to be done with many thin layers. You tint the filler first so it matches the leather. Then sand it. Put the shoe trees in the boots every night so the shallow creases don't become deep again. It's quite a big ask to expect the filler to stay in. Best to replace the boots with lightly used ones from ebay. Prices are often v. v. low. If you're wary of fungi from the previous owner you can kill them with UV light.