Dogs jumping up. What to do?

I think what you're doinng is fine, you could also say "down" when they do jump on you.

Just a suggestion, but if it's a small park only used by locals maybe you could ask owners not to feed others' dogs treats whilst in the park,only their own dog as needed for training etc. as it does encourage this behaviour and other park users may be scared of dogs - not just you.

Have to say most dogs who approach me in local park are hoping for treats - if I show them empty hands they soon lose interest.

You could try this - as the dog begins to jump up at you, just lift a knee. You’ll catch the dog on its chest, knock it off balance and it’ll soon get the message to stay down,
I hasten to add you aren’t kicking the dog, just placing a barrier (your knee) between them and you,

Just turn away from them as they approach you, they don’t tend to jump up unless they see you are looking at them?

Down for my dogs means lie down. It would for the majority of well trained dogs. Off is the command if they jump up ( my sons in law aren’t as well trained as my loooy lab and hug him if he leaps for joy when they arrive)

I would not be patient with the off lead, badly behaved dogs jumping up at me, it’s totally unacceptable, a small water pistol right in their face won’t hurt but might shock. It’s not your job to ask owners not to treat other folks dogs. It’s very basic dog ownership responsibility
There’s a k9 spray. Legal and available on Amazon.

Who owns the park, is it insured? If the insurance policy states dogs should be on a lead the insurance is likely invalidated if they are not.

Firing a water pistol or dog deterrent spray at a dog may cause aggression (from the dog and/or it's irresponsible owner) don't risk it.

The owners need telling, especially if the park is shared with children, sounds like a bad incident waiting to happen.

On the beach dogs can jump you. And as I have dogs I think it makes me more attractive. I can be a little unstable as as I have two new hips. I have a walking pole which I plant in front of me and dogs go round. Perhaps some people have hit out at lively dogs which I have never done but it certainly does the trick

I use an open hand blocking gesture before the dog reaches me, it usually works.
Dogs respond more to body language than actual language.

I love dogs ,but I dislike the owners of irresponsible dog owners.
I hate dogs in cafes and restaurants especially when they feed from the table or have treats put on crockery on the floor .

I understand your reservations .
Not everyone wants dogs jumping up at them .
At home , my father used to bang his newspaper on a wall or furniture and shout no and get down.
It's not the best way to train them !
I'd have three or four of them and they had to observe some rules otherwise it was chaos .
I've folded my arms and turned my back on them then when they obey sit they get a reward.

This isn't do easy in a park .
I'd be tempted to get some notices done as you are the key holder for the park .
The notices could outline what is acceptable and what isn't.
Something like :
We welcome well behaved dogs in our park.
Please muzzle your dogs if she or he nips.
Please pick up all poop ,bag it and put in the bag provided.
Please don't allow your dog to jump up at people unless invited - not all of us appreciate it and some of us are unsteady on their feet .

Thank you.