Our apple tree wasn't doing much of anything (it barely even ever flowered), so I finally cut it down and bark grafted 3 different varieties onto it - Liberty, Ashmead, and Cox Orange Pippin. These should all be able to pollinate one another. We also have a Honeycrisp tree nearby that should benefit from this, too.
I have an Apple tree about that size that only makes good cricket balls. I've never done grafting before - as there a good guide you followed?
There are a lot of YouTube videos showing various types of grafts. I would look there first - I learned from watching videos. I also reference this a good bit: https://www.wikihow.com/Graft-a-Tree (but you can't beat watching a video of someone do it).
For my tree I did what's called a bark graft, which is often used for trees that stop producing or to change the variety. There are also bud grafts, whip and tongue grafts, cleft grafts, etc.
Be sure also to get varieties that are good for your growing zone and that flower around the same time so they can pollinate one another.