On the first attempt accessing the website, it keeps loading on Cloudflare checking I'm stuck there until I disable the desktop mode now I can access the site.
The design looks fine to me it was a dark version before and now it's a light version but you can switch to dark mode if you want under the menu or top-right corner.
All buttons are functional and fast same goes for the Tor browser all of them are functional. The page size is 80kb according to some checkers and the performance of the sites is great.
About "LetterGuarantee.txt" it shows the date, deposit/your BTC address, service fee, PGP fingerprint/signature, and warnings. The format is just the same as other mixers out there.
The mixing process on basic anonymity is smooth after waiting for a few hours it takes hours for me to receive BTC but I think the issue is due to network(transaction fee) spikes recently. Other options are a bit expensive so I can't afford to test them.
The only downside of this it uses a cloud flare which we know records the IP address of their customers/visitors. So I'm not sure but since you said you don't keep logs how about the Cloud flare? So I think better bring your customers to Tor link instead to avoid this.
Another thing is there are some issues with the website related to accessibility you need to fix them look at this link below.
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https://pagespeed.web.dev/analysis/https-webmixer-io/20r3dwjxjt?form_factor=mobile