About ten years ago, I went through this process with as many data brokers and reporting agencies as possible. I printed documents, made accounts, mailed letters, and jumped through every hoop they set up. Some of the companies only recognized my request for a term of five years, while some were permanent.

I don't get random credit card offers in the mail any more. However, since I started using an email alias service a few years ago, I've found my data making its way back to the data brokers. I'm not in the EU, so I don't think I have much recourse for it. Even so, I think it's worth it to jump through all the hoops. Maybe one day it'll feel more meaningful.

tl;dr: It was cumbersome to do it, and other than a reduction in snail mail spam, I haven't seen much benefit from it. All the same, I'd do it again.

I have had a lot of success.

First I went through this list and filled out lots of opt-out forms: https://github.com/yaelwrites/Big-Ass-Data-Broker-Opt-Out-List

Then I emailed everyone on this (dated) list: https://github.com/privacybot-berkeley/privacybot/blob/master/app%2Fservices_list_06May2021.csv

I got a lot of follow up emails suggesting that my requests were honored, or having me do follow up steps to have my request honored. Only two pushback emails suggesting that they don't have legally respect my opt out, to which I provided them a strict do-not-contact request that hopefully encourages them to do nothing with any data they might have on me.

It was a huge amount of effort (~50 hours), but my private info was nearly ungooglable a couple of weeks after I finished.

After seeing a huge difference, I then signed up for DeleteMe, and compared to a few friends of mine who never did their own work before signing up, my quarterly reports are extremely sparse. Hopefully it's easier for them to play whack-a-mole with my data since I did lots of the initial work.

So far I can't find anything about myself I wouldn't want to see on any search engine. If I did, I would just find the opt out page and try to nip it in the bud.

it feels like I’m just giving my data to them

Same, OP. I wish there's a way to just give your name and have them delete everything they have on that, instead of possibly giving them even more information than what they had just to request to opt out.

Discover Bank (and I think credit cards) have a feature where they'll monitor certain data brokers for your info and then submit block requests for you.

Discover Online Privacy Protection

You should do that with your municipal administration.

Otherwise, i think it makes more sense to prevent them to get the data in the first place.