First of all, welcome! It's always great to meet another toku fan. Also, apologies in advance for what I'm sure will be a reeeally long post. :laugh:
Garo and beyond
When you say you enjoyed Garo I'm not sure whether you meant just the original series or the big franchise that has spawned from it. There's a lot of content there (admittedly of varying quality) but it's well worth exploring if you've only seen the first series.
Garo was created by a man called Keita Amemiya. He's well known for having worked on a number of famous (and infamous) tokusatsu. It would be very easy to just quote big chunks of his resume at you, but for now I'll just mention his two most "Garo-like" shows and say that if his name is attached to a project then it's likely to have strong visuals.
Tekkōki Mikazuki is a six episode mini-series about a boy piloting a giant robot against monsters. You can see it's where Amemiya was developing the style he'd later use in Garo. Shougeki Gouraigan takes the visual style and action of Garo but adds some humour and focuses more on the relationships between it's characters.
Ultra series
You mentioned that you're familiar with kaiju films

thumbs

. I'm assuming you're also familiar with how Godzilla tends to set the trends that those films follow. Whatever direction that franchise has taken over the years has tended to dictate what other kaiju films do.
Why am I talking about Godzilla in a section about Ultraman? Well, as it happens the creator of Ultraman was the guy in charge of the special effects on the original Godzilla, and ran Toho's effects department through to the late 60s. As such, the Ultraman franchise has tended to follow the same trends that kaiju movies have been going through.
So if you like a particular era of kaiju movie, chances are you'll like the Ultra series being made at that time. If you're not so keen on the films made around a certain time, you may not be so interested in those Ultras either.
However, for some examples from each era, see below:
[HIDE]Let's start with the
2000s:
Ultraman Nexus had a move prequel that was known simply as 'Ultraman' in Japan, but is usually referred to as 'Ultraman 2004' or 'Ultraman The Next' to differentiate it from the original 60s TV series. The film is available via fansubs.
UltraSeven X is very much like a mix of elements between Nexus and Garo (it's long been rumoured that staff from Garo were hired to work on it, though I've never seen confirmation of that). Well worth checking out.
While it is more "family friendly" than the shows above, it would be remiss of me to not mention my personal favourite, Ultraman Mebius, which is available to stream for free on Crunchyroll.
From the
90s, Ultraman Tiga is held as one of the great Ultra shows and has been fansubbed. Gaia is also available on Crunchyroll.
Ultraman sat out most of the 80s. However from the original
60s-70s run I'd point to UltraSeven, which is available to stream in America from Shout Factory's website, or the fan-favourite Ultraman Leo, which can be streamed from Crunchyroll.[/HIDE]
Side note: You may also want to check out a (sadly now ended) youtube show called Scifi Japan TV. It interviewed people who worked on classic Toho kaiju films but also did some Ultraman features, including a three-part behind-the-scenes special with the creative staff of Nexus.
Kamen Rider
One of the big three 'henshin hero' franchises along with Ultraman and Super Sentai. The Rider series have probably varied the most in tone over the years, and are very easy to get hold of. In your case, I'd maybe pluck for Kamen Rider Kuuga or Kamen Rider Amazon
s (please note the plural, as there is an older Amazon that is a very different show) which are good examples of Rider at the more Nexus-like end of the scale.