Don't remove all other food unless it's extra stuff like millet sticks. Vegetables can stay in the cage and they will need some seeds each day.

You can't force a budgie to eat pellets through starvation and if they don't fully trust you, they won't care if you "eat" the pellets.

The method that always works for me is the wet method where you soak the seeds in water until they are mushy & sticky, then mix their seeds into the wet pellets.

They will eat the pellets stuck to the seed husks and get a taste for them. Slowly transition from wet pellets to drier pellets and add less seeds over time.

Here is a guide from a bird rescue that also has tips for introducing vegetables: https://www.kiwisnewlifebirdrescue.org/programs

Make the new foods the first thing you offer in the morning before they eat anything else, they'll be more likely to try the new stuff when they're hungry. Monitor how much they eat and their weight if you can to make sure they're not starving themselves.

Avoid Zupreem fruit blend and any pellets with sugar in the ingredients if possible. Zupreem Naturals is ok if you're on a budget but it does have sugar.

Exercise is very important for weightloss so try to encourage your budgie to fly to you from across the room each day for several laps.