TBH these are not a good set of dimensions for a 60” or 5-foot tank. I would have certainly gone the standard 5’ x 2’ x 2’ (60 x 24 x 24) plus a 24” tall instead of a 30” tall is much easier to reach and clean for many people. Having said this, I’m not sure about most predatory fish like eels and that, but assuming a 5 x 2 x 2 I think you could do 2 Tangs, I’d pick a Zebrasoma (excluding the Sailfins) and either a Bristletooth or a smaller Acanthurus species like the Lavender.
As for other fish, I’d definitely be keen on a few angels. In fact if given the choice between foregoing Angels altogether or foregoing Tangs altogether I’d choose Angels any day. A 5 x 2 x 2 would be able to fit the following:
Smaller Genicanthus species (Spot Breast, Watanabei’s, Bellus being the accessible ones)
Any Centropyge / Paracentropyge
Smaller Chaetodontoplus (Blue Spotted, Singapore, Black Velvet)
Maybe something like an Apolemichthys too (Bandits are inaccessible for the most part because- Hawaii, but I think a Gold Flake which is being captive bred could make a great showpiece fish)
You could also do smaller butterflies (subgenus Roaps, Prognathodes) and a Copperband if you find a healthy one and you’re lucky enough to get it to eat and put on weight.
If this was my tank and I had the full range of CB options available to me, I’d probably get a pair of Coral Beauties from Biota, have a Gold Flake as my showpiece fish, a small Genicanthus or smaller Chaetodontoplus, and fill the rest with smaller fish like wrasses, basslets, damsels, Clown pair and so on.
However, I do think 2 Tangs is about right for a 5 x 2 x 2.
But I’d urge you to consider 60 x 24 x 24 for a start rather than 60 x 18 x 30, the 18” width is awkward.
Edit: You might want to consider the RSR 650 which is 60” x 26” x 24” and gives a volume of about 155 gallons.