Has anyone used them?

My question to you is why you decided to choose them and invest your hard-earned cash.

• A quick search shows no reviews online.
• Looking at their website, they say they are in the UK and the address shows as Austria 

• The domain was registered on 15/06/2022, so they have been around 5 minutes.

So I would be nervous.

But as for time-wise, SEO is a longterm investment, not an overnight magic bullet, but you get what you pay for. So you should have been given realistic expectations, and then had goals and targets you can measure, so you can decide if it's a good investment.

In 4 - 6 weeks I can confidently rank a low competitive keyword and have it in the top 3 in Google. The more competition, the harder the task and the longer it takes and the bigger budget you need.

 It largely depends on how competitive the market is and the number of keywords you target.  However, 4-6 weeks does not sound unrealistic.

I can happily look at your site if you want a second opinion.