How do I make a custom form style be the "default" style for my invoices? The New invoice "Experience", defaults to "Modern" and has very limited options to customize.

I understand the challenges you're experiencing with the custom form styles on the new invoice experience, Teddyb. 

I'm here to assist with how to make your Custom form style the default style for your invoices and have the Account summary on it.

Go to your invoice template. Then, from the Content tab, ensure to tick the Show on invoice box. Tap Done.

Afterward, choose the custom template you wish to set as the default option from the Design dropdown. Subsequently, select the Remove default hyperlink to activate the new custom template as the default invoice template.

In addition, I'm including an article that offers guidance on creating customer statements to display overviews of their invoices, payments, credits, and outstanding balances: Create and send customer statements in QuickBooks Online.

Keep me posted if you need further assistance managing your invoices in QuickBooks. I'll be here to provide you with the support that you need. Keep safe.