Now I understand it is a pandemic, but most hotels around Europe have opened up and are desperate for customers now. For example, me and my partner want to go to Rome on the first weekend in October. When I searched through the Marriott Bonvoy website, I find 12 hotels only! 12! When I search through Booking.com - I find over 2,000 4 and 5 star properties.
You're not using Booking.com correctly in terms of comparison. Unlike Marriott.com or other hotel group websites, many Online Travel Agencies (OTAs) like Booking.com include other types of lodging.
After checking the 4 and 5 star boxes, keep scrolling down on the right. (That's on the full version. No clue how their mobile site is set up.) Eventually, you'll get to the Property Type section. it looks like this:
Source:
https://www.booking.com/searchresult...r_ht_id=1&rdf=
Those are the number for Rome, by the way. For an accurate comparison, you need to select the "Hotels" box and ignore all the other options.
When you do that, those 12 Marriott hotels suddenly become roughly 5% of the 235 hotels that are rated 4 or 5 star in the city. If that's not enough choices for you, perhaps you should follow
craigthemif's advice and just book the best option regardless of hotel group connection.