Well, the good news is that it sounds like you've found a good hotel that treats all of its guests well. That's the way it should be - and IMHO this is one of the strengths about the full-service Marriott brand. Consistency and generally good service for everyone.

The downside is that (although there's a slim chance that I'm wrong) one trip per year booked through a third-party agent isn't going to have any effect on the treatment you receive. In fact, I'm not 100% sure they'd recognize that you'd even been there before based solely on your name and perhaps address (not sure what data is stored in the system for a third-party booking).

Repeat business on the order of weekly or monthly visits, direct booked with a Marriott Rewards number, can definitely serve to augment the benefits you get from the program status. But it's done in subtle ways - for example, hotels will get to know your detailed upgrade preferences and things like that. Once in a while they might comp a bottle of wine or otherwise give you a small goodie. But IME that's only been done after a series of frequent stays, all booked via Marriott.com, as a Gold member.

Most big hotels have a lot of repeat guests, so I understand that even if I do 25 nights a year in one place, I'm pretty far down on the pecking order.

EDITED TO ADD: As long as you have a use for at least an occasional Marriott stay, you should pick up the new Marriott Premier Visa. The certificate you get each year for 1 night in a Cat 1-4 (or perhaps 5?) hotel is easily worth the $65 annual fee, plus you get enough bonus night-credits added to your account to maintain perpetual Silver status. (Silver is a somewhat weak level, but it's better than nothing. )