Hi OP - jumping on to say this sounds pretty normal in my experience too! My spaniel/lab cross is a horror on the lead if he's smelled something, e.g. a cat, and if it's first thing in the morning I just hang on for dear life. He's getting better at stuff like heelwork when he's calmer, but he's eight months old, and when they're this sort of age they don't have a lot of impulse control. The more time off lead I can give him, the more sane he is afterwards, though he's still fairly barking as it were.
I started with a 5m long line, dropped the end and ran away from him to get him to chase me, then added a cue, a hand signal, then a whistle. He's a bit dodgier with recall now he's a teenager, but you only need enough to let him run round a bit and sniff to his heart's content. Perhaps a 10m long line would give him a bit more sniffing without losing him completely?
Also, even if it's just a boring secure field you can throw treats in the grass and get him to hunt them out. I'm sure you've tried different types of treats, but just in case, Pooch and Mutt do lots that have probiotics which are good for their systems and which don't have things like chicken or beef or lactose, and I chop them up even smaller; or you can get things like rabbit or goat paté from places like Paws Trading and Anco, which dogs shouldn't be allergic to, and again, chop them up into teeny bits.
@GuppytheCat that scentwork sounds amazing. Is there anywhere you could point me to learn more? I know my hooligan would love that.