If you're looking for quick and simple... I find that Espoma is a good slow release product. Whatever you get, you'll need to find out the balance of K that will work for your needs.

I maybe misunderstanding what you are trying to say. Did you say that no one but Osmocote sells a slower released fertilizer in quantities needed for anything about a 20 square foot plot ?

If so why not use Osmocote? I happen to like it and on Amazon you can find it in 50lb bags ( granted it's pricey IMO ).

But do you need a lot of it? I didn't think you need a lot. I would probably use composted manure . Rock Phosphate , bone meal and manure are all sources of phosphate.

I have a 1600 square foot garden. If I use Shake and Feed, I need like 10 canisters!

I am using bone meal as a general P supplement, but wanted something extra for side-dressing etc.

Convincing the spousal unit that it's worth spending $100+ on a big bag of Osmocote is the challenge.

Not sure where you are looking but there are any number of brands of 10-10-10, 13-13-13, 5-12-10, and all sorts of number combos available in 40-50 lb. bags sold at Walmart, Home Depot, Lowes, Menards, Farm & Fleet, Tractor Supply, and similar stores for $12-15.

If phos is all you want then bone meal and rock phosphate (also sold at those same stores) in big bags. Fish bone meal is another. There have been a couple of other discussions here just recently about organic sources of phos that the search will pull up.

Also your soil test should have provided you with instructions for what and how much to use.

Dave

https://www.walmart.com/ip/Expert-Gardener-40LB-10-10-10/166185332

I have discovered crab shell fertilizer and its amazing stuff!!

Check it out

https://youtu.be/A6mDlAUTCUI