A persistent fungus gnat problem, which can indeed be tricky, especially in indoor settings. Some effective strategies 1. Watering Practices: Fungus gnats thrive in moist environments, so the key is to adjust your watering practices. Allow the top inch of soil in your pots to dry out completely between waterings. This practice will deter females from laying eggs in the dry topsoil, reducing the larval population significantly. Overwatering is a common cause of gnat infestations, so be cautious with how much water you're using.
Consider covering the soil surface with a thin layer of coarse sand or fine gravel. This will help keep the top layer dry and also deter gnats from laying eggs in the soil [¹].
You can use Bacillus thuringiensis subsp. israelensis (Bti), a biological control option that targets the gnat larvae in the soil. This natural bacterium is safe for your plants, pets, and humans. Products like Gnatrol or Mosquito Bits are often available at garden centers and should be applied according to the label instructions.
Sticky traps as they are effective in catching adult gnats, which can help in monitoring and controlling the population. Ensure that you have enough traps positioned near the affected plants.
Inspect your pots for any debris or algal growth that could be harboring larvae. It’s important to keep the surfaces clean by removing any dead plant material around your herbs and lemon tree. If your plants are in trays, ensure there is no standing water.
 If the situation does not improve, you may need to consider more drastic chemical controls aimed at gnat larvae in the potting soil. Always choose products that are safe for use on edibles and follow the manufacturer’s instructions carefully. 
Quoting from the UMN web page:
"Cover the soil with a thin layer of sand, diatomaceous earth, or fine gravel. This barrier makes it harder for adult gnats to reach the soil and lay eggs. A layer at least half an inch thick is best."
Information regarding pesticides, herbicides, or other chemical products is intended for general guidance only. Always refer to product labels, consult certified experts, and adhere strictly to local laws and regulations before using any chemical products. See also

https://extension.psu.edu/fungus-gnats-in-indoor-plants#:~:text=Fungus%20gnat%20larvae%20usually%20are,fungi%2C%20and%20decaying%20plant%20matter.&text=Gnats%20are%20not%20strong%20fliers,nor%20do%20they%20bite%20people.

https://extension.umn.edu/yard-and-garden-news/how-treat-pesky-fungus-gnats-houseplants