You can write your own frontend for Lemmy by connecting to the Api and using it to build the HTML. The docs have some more explanation.

You can do some pretty cool customization, but I have no idea how. I came across this lemmy instance that looks like an old phpBB forum:

https://fedibb.ml/

I'd just like Lemmy to take up the full width of the screen. I hate this design trend of wasting 50% of your screen real estate. Old Reddit scales horizontally pretty well, especially with a plugin to hide the sidebar which makes it work well on portrait and landscape on mobile. Lemmy is cluttered because of how narrow it is. I messed around with the developer console and made it full width and it looked a lot more usable, but unfortunately those changes aren't permanent and I don't know how to make a user style or browser plugin to do this change myself.

On Android, Jerboa has a "list" style post view that may be more to your liking that the default card views.

There's no multireddit feature currently. I don't have a link handy, but there's a GitHub issue for it though.

In general, I think you'll find that Lemmy lacks many quality of life features you love in the reddit ecosystem. It's a younger project, and less well funded. If you have a developer background or ability to learn, you may be able to help... but it doesn't have the huge variety of apps and interfaces that reddit does, and you're not necessarily going to be able to replicate a highly personalized UI experience terribly soon.

It's an open issue on their github. The main devs are way too busy to deal with it. I'm trying to grok as much Rust & Psql knowledge as I can to be able to contribute, but it'll take a while. Anybody with Rust & Psql knowledge can contribute, the devs are open to any contribution to improve the platform

Oh man, the grouping feature would be especially amazing in the context of the fediverse. I'd love to group all my News communities into one set so I can browse just news for instance. I see it like clicking Subscribed with a subset of All, custom group 1, custom group 2, etc.

You could check out kbin.social and see if the customization options can be made to your liking. You can definitely turn off thumbnails on a user level for example and make it look quite old-reddity in general.
Would need a new account of course, but thanks to federation you can still follow and participate in the same communities.

Themes would be my start, but that would require the instance owners to add them once created.

I've been working on an alternate frontend that might fit your needs.

https://github.com/rystaf/mlmym

Here's a link to this post on the official instance

https://mlmym.org/sh.itjust.works/post/5401

I actually would really like something like new Reddit, Lemmy is far too horizontally dense despite wasting tons of space on both sides. I can't actually tell which comment is the top most either.

Generally its just difficult to discern what exactly I'm seeing.

If I don't find something soon-ish I'll probably get deep into making my own restyle script.

If you have the stylus extension, https://userstyles.world/style/10168/1440p-lemmy is an improvement, but I'd like to see some further/other changes or options.

On lemmy? No. The devs decided to do a single page app with a REST API, which leads to the sluggishness and slow load-time. kbin (which is compatible with lemmy's content) is your best bet, IMHO. https://kbin.fediverse.observer/list

I'm too used to lemmy unfortunately, and kbin is far from perfect.

already exists. https://old.lemmy.ca I was surprised to learn how many different front ends my instance already had. I'm not sure what it takes to get it integrated on your instance