K1 vs Marriage Visitor Visa, should we seek legal advice?

Myself and my partner plan to marry. I’m British, they’re American (naturalised), and we’ve been together almost 3yrs.

Toyed with the idea of a K1 but seems like the fees for MVV are less, though we would need the visa for my partner to visit before we can give notice, then book another visit within 6 months.

I’ve seen on here people say it’s legal to get married on a tourist visa in the US as long as you don’t apply for adjustment of status, so then, if you do intend to settle you must go the K1 route? Both seem v complicated and every time I feel like I’ve made a decision on which one is easier/cheaper I get scared we’ve overlooked something. Advice?