Best way to gain customers manually?

You'll have to get creative.
On twitter use advanced search to find people asking "which landing page builder" or "I'm struggling" + "Landing page"

Offer to make a new landing page for people, use your tool. record it in loom. and then post the video to youtube. label it "landing page for <topic>" if you find 20 people with different topics, Post them all to youtube. Count the views 2 weeks. YouTube will tell you which topic has more "views" hence... people searching.

Search through indiehackers, find people who have a landing page, and also post about struggling... make a new landing page for them. Send them the video of you using your own tool.

In general, there is no "best" way - it depends on your project and audience. That's also part of the reason you see so much advice about finding your audience before building a product - maybe your product is relevant to an audience, but how many people think it's a critical problem and want to spend money on it?

Anyway, at $55/mo, you're better off than me, so I won't pretend to lecture you :-D

On to your product:

In general, there are a ton of theme sites available, so you need a clear selling point that matters a lot to purchasers. Which probably means a lot of views, and designs that really stand out to some segment.

IndieHackers is a tough sell in general. We're struggling to make money on our own products here, so spending money on other products is not what we're here for, unless it's going to save us a lot of time or money.

Reddit is just tough in general. General consensus is they hate self-promotion unless you're giving free stuff away.

E-mail could be great if you find people with ugly websites and reach out to them. But also, it could be difficult since they're not always the most technical.

I think Twitter/IH could be good (IH also includes email - gotta reach them somehow), but you have to work hard to personalize your message to each person.

I've personally ranted a tiny bit about how themes are overly animated lately and I think it detracts from the message, so I'm obviously not your target. But that is literally one person's opinion. Maybe your pages appeal more to creatives, or I'm flat out wrong!

Side note: I loved your video. The soundtrack alone made me keep watching it ;-)

Email, don't look any other way. If I were you, I would target web developers - preferably freelancers. A simple email like this would work great:

==
Sub : Collaborating with <XYZ Agency>

Hello Mike,

I am Brayden from BuildFaster. We work with web devs like you to help you build client websites faster while also saving costs on web design.

I would like to offer you an agency discount if you use our themes for your upcoming dev projects. Would you be interested?

Thanks,
Brayden
==

I noticed that your themes are really cheap. Price them at least $30 to $40, and then offer them for $10 to developers. Should work IMO