My kids loved sticker reward charts. You can get rewards charts and stickers in some of the buggies pull ups.

yes the undies idea is great! you could present them with a “big kid pack” with a potty, undies and a magazine for them to read while on the potty lol. it could help them to get excited

I got my kids all to choose ‘big kid undies’ and they were so excited and then I put undies under the pull-ups so they feel the wetness more and they soon understood that they didn’t want to dirty their new undies so they would run to the toilet.

just get them used to the potty in the bathroom and then gradually encourage them to sit on it and then eventually use it. let them take their time.