Lemon Tree with Stunted Growth...

Recently bought a house with a fairly established lemon tree. This is my first fruit tree.

I pruned it back pretty far in early spring - I don't think it had ever been cut back before.

It started coming back strong with large, beautiful green leaves. It grew strong for a couple months, then it just stopped.

The leaves are now have a slightly yellow color to them and, while still firm and strong, they point downwards. I have tried many things to get it going again - coffee grounds, fresh mulch, extra water, straight coffee, eucalyptus mulch for the acidity...nothing works.

I did spray DE on the tree when the leaves were being eaten by something a few months back. Could that have stopped growth?

I'll get a picture up once the thunderstorm stops.

Quit with the Eucalyptus mulch and remove as much as possible, including a few inches of the soil under the mulch.
Many Eucalyptus produce chemicals that stop the growth under the trees, and the leaves and chips of wood that you are using for mulch are releasing these chemicals to the soil.
The fancy term for this is allelopathy.

Citrus do not always do well when they are pruned hard. Better to work your way through the plant over time, a little now, a little more after some recovery has happened in the first area. The other concern is if you cut it all the way to the ground, or the pruning triggers a growth response from the root or low on the trunk. Citrus are grafted and the growth that comes from below the graft is not good for edible fruit. Watch out for growth coming from too low on the plant. Usually this growth will have smaller leaves.

Citrus need plenty of fertilizer to put on such rich, fast growth. Use a complete fertilizer with N, P, K and trace elements. You can use organic materials like blood meal, worm castings or other things.
Too fast, too lush and the tender growth will not be resistant to pests. It can actually attract them. Severe pruning and too much fast releasing fertilizer will trigger this sort of growth. Best is to apply a slower release fertilizer (including organic materials- most of these are slower release) so the plant gets a more constant feeding at a lower level.

Mulch with any sort of bark chips or ground bark that is sold for mulch, or you can use most recycled materials such as tree company chips, but make sure it does not contain plant material like Eucalyptus or Walnut. There are a few other plants that reduce the growth of nearby plants. If you get the chips from a licensed arborist he or she will know what plants are likely to be in your area and cause such problems. (and whether the company has pruned any of these in this load of mulch)
Any sort of mulch that was originally plant material (bark, wood chips, manure from herbivores) will have an acidic reaction in the soil, so unless your soil has extremely high pH then pretty much any mulch will gradually lower the pH.

Instead of using abbreviations, spell out what you used.
Is DE Diatomaceous Earth?
I do not think this has harmed the plant.
Citrus do have a few pests, and if the pests are eating enough of the plant to be a concern then spraying is warranted. A few nipped leaves here and there may not be as much of a concern. Did you ever find or ID the pest?

A soil test might be a good idea.
You would then know:
a) pH and why it has that pH. What to do if correction is needed.
b) fertilizer needs. (ie: if your soil is already high in certain nutrients, no need to add more. Conversely, if the soil is particularly low in certain nutrients, then you know to add those)
c) other issues.
There are kits you can buy to test N, P, K and pH. I would start with that, and see if the corrections suggested by these tests correct the problems. These kits are pretty cheap, and may not be perfect, but it is at least a start.
To get a more complete analysis you can check at university extension services, department of agriculture and similar places. These more complete tests cost more. Perhaps just speaking to one of the experts in these offices would be enough. They will be familiar with your area and will be able to tell you if the local soils tend to have specific problems.

Please post some pictures of the leaves.
There are different ways plants can go yellow, and these patterns can be a key to nutrient deficiencies and other problems.
A picture of the whole plant, and close ups of the leaves would be good.