I'm new to this plant and so have no advice for you except as long as the slow release is in the mix, probably best not to fertilize otherwise, of course. I understand you are looking for a more successful blooming season, so hopefully it will happen. Very pretty plant with a balanced appearance in an attractive pot.

Adeniums are full-sun trees … The more sun, the healthier the plant and overall blooming. They are also heavy feeders. More than anything, they need consistent feeding. You CAN use a slow-release fertilizer every 3 months AND a liquid fertilizer at 1/4 strength.

My adeniums (80+) are in full hot Caribbean sun and get full rain. Remember that potted plants have nutrients leeched out of the soil when exposed to downpours. I use any complete powdered fertilizer at 1/4 strength (to minimize salts added to the soil) and rotate between adding the water of boiled banana peels, soaked & strained aloe gel and garlic skins, earthworm castings & castings tea, fish emulsion. These are all very effective fertilizers on their own.

You can fertilize weekly at 1/4 strength, every 2 weeks at 1/2 strength, or monthly at full strength.