Get a magnifying glass or loupe 30x is a good one but 10x should still work. Look under the leaf and see if you see anything on it. Thrips, whiteflies, aphids, could all cause light areas on the leaf like the one you are showing. So could over watering.
If the lemon has bright light it can stay indoors. Please update your profile with your zone and location. It helps us when you ask questions in a post and you won't have to keep telling us all the time. If you live in a frost free zone, the tree will be happier outside.
It is hard to water a pot indoors because it dries out slower. A five inch pot to a 24 inch pot is a big jump for a plant. If the pot is too big it will take longer to dry and citrus likes deep watering but well drained soil. I would have gone to maybe a 1 or 3 gallon pot at the biggest from a 5 inch pot. Citrus like well drained acidic soil with a pH of around 6-6.5.
Citrus trees do have growth spurts, that is normal and they usually do not drop all their leaves.
I would check the roots and see if they are healthy and not black or soft. If they are either, you have no choice but to try to repot in a smaller pot. Cut off the dead roots with a clean pruner. Depending on the size of the root mass, I would just get a pot that has maybe 2-3 inches larger in diameter than the root mass. Personally, I don't like to repot citrus trees often so I do something that most people will not do. I plant all of my citrus trees in cinder. Air space and drainage is the most important thing for keeping citrus happy in a pot for a long time. Cinder allows for good drainage so don't mess it up with a saucer. No saucer under the pot. If you have to use the saucer, turn it upside down and use it as a pedestal. Cinder has no fertilizer. I use citrus food, Vigoro, I get it from Home depot, but any citrus food will do. Read the label, most times the fertilizer application depends on the diameter of the trunk. For a 1 gallon pot, 1 tablespoon in the spring with the first leaf flush and another feeding at fruti set at the end of summer is enough. If you want you can supplement with miracle grow for acid loving plants 1/4 strength once a week. (Weakly, weekly feedings). You don't have to use cinder, but it needs to be an acidic well drained mix. You can add some organic like peat moss or coir to a cactus mix to hold water longer. Cactus mixes tend to be alkaline so add a little bit of sulfur or use miracle grow for acid loving plants. I contains sulfur so it is an acidifying fertilizer. Black cinder is neutral. So is blue rock, but blue rock is much heavier and has fewer air spaces. If you want to make your own mix:
1 part peat moss or coir
1.5 parts number 3 or 4 perlite (large perlite, not the fine number 1 or 2.)
How often to water depends on how fast it dries out. If you can't tell and it can be hard to tell with cinder, get a moisture meter. Overwatering houseplants is the number one problem with most houseplants, second is not providing the right kind of light. Getting a combo meter can really help with those issues.
https://www.amazon.com/Moisture-Meter-Li ... B007FMVOVK
I call it the plant happiness meter. Mine is more for fun and it is not the same model as this one. This is just one that I found on the search. I think it just has a guage. Mine makes a sound; the faster the sound the happier and wetter it is. When the plant is dry the meter sounds off very slowly. The light sensor, I use to determine if my orchids are getting the right light. It is more of an empirical way to judge light. The other way is to look at the leaf texture (soft and dark green is too little light, Yellow or red and thick leaves means they are getting more light than they need, medium green, firm leaves= just right). You can use the light meter on a camera too. 10,000 lumens is full sun, 8,000 lumens is about 35% shade, 5,000 lumens is about 50% shade, 3000 lumens is 70% deep shade. Moonlight can be as low as 0.25 lumen. A lumen is the amount of light that reaches an object or foot candles per square foot. The light source is usually measured in foot candles of output.
