Here’s a post with the same issue and solutions🙂.

I know you said the soil is fine, but it’s a good idea to microwave the soil when planting succulent seeds. Especially if you are sealing the newly planted seeds in a bag.

You don't give any description of the pests infesting your plant, so I'll guess at fungus gnats. Anything that might infest dry seed is unlikely to be a pest in actively growing plantings; and, since FG eggs can lie dormant in dry soil for a long time before moist/wet conditions stimulates the hatch, so the soil might still be the source of the infestation. I suppose at this point it's best if you confirm whether of not you think it's FGs at work, or if it's some other pest ...... and a detailed description or good image wouldn't hurt your cause at all. If it is a FG infestation, there are several ways to remedy it. One way is a spritzer filled with 3 parts distilled water and 1 part 3% H2O2, which is the hydrogen peroxide you can buy at all drug stores, dollar type stores, grocery stores, beauty supply stores (check the % on the label from BS stores), and other less expected places. The same solution can be used to sterilize the seeds - only takes a 2 minute soak.

BTW - when starting seeds using a water-retentive soil, you might see an infestation in shallow containers but not in deeper pots.

Any given soil, if it supports perched (excess) water, will support a perched water column of the same height in any given container. If your soil is 3" deep and your soil supports a 3" perched water column or more. after a thorough watering the soil would be 100% saturated - heaven on earth for the little bugly uggers.

Al