In that case I think it is just slightly more convenient to render a PCI compliant entity's frontend as an iframe inside your application and have it send the details directly to the processor's server, perhaps saves one additional hop
In general I am a fan of just using things like Stripe Checkout though - it's not just about compliance but also just the complexity of getting the frontend part right with the myriad different payment flows, errors, and other subtle details that exist.
But my original comment was more just clearing up confusion that you can absolutely do the frontend UI customly if you want to - there's no issue with this and PCI compliance, that only kicks in once the data goes to your server.
Have you considered using Stripe Elements? It has loads of customization options that you can configure using an appearance API