What's your Marriott status? There's a status match to Silver for Marriott Bonvoy Titanium members ("Ambassador Elite and Titanium Elite members receive complimentary United MileagePlus® Premier® Silver status."). If you're not Titanium then that would rule out United as an option.

Qatar Airways use Avios which are some of the easiest points to get (with very few limits). The Qatar Avios system integration with British Airways is a little cumbersome but you can instantly transfer Avios between British Airways and Qatar Airways. You can buy Avios direct with Qatar Airways but I think it's a better deal through British Airways normally. I think the flow would be: set up a British Airways account, buy some Avios, set up a Qatar Airways account, transfer Avios from British Airways to Qatar Airways, then transfer to Marriott Bonvoy. The transfer rate isn't as good as United unfortunately and less generous bonuses so it is a much more expensive route, I think you would need to pay ~$6k for 100k Marriott Bonvoy points when going via the avios route.