There's several ways to look at the differences.

The one that will jump out at most programmers who are familiar with mainstream languages is that J, k, q and Nial use ASCII characters while APL, BQN and Uiua prefer glyphs. q and Nial additionally favor words rather than shortened abbreviations, and Uiua has plain words that auto-format to its glyphs to aid in typing. The other glyph-based languages rely on custom (software) keyboard layouts or input methods to let you type the symbols they need. You do not need a special keyboard to program in any of these languages. ASCII-or-not is not a decision that any of the array languages have made lightly or for purely aesthetic reasons, it has deep consequences for how the languages feel that won't really make sense until you get some hands-on experience. As a beginner you'll probably gravitate towards one of the sides without understanding those deeper implications, and that's totally okay, but please keep an open mind.

If access to a high-quality open-source implementation is important for you, your options narrow a bit. J, BQN, Uiua and Nial all have a primary implementation that's open source. k has implementations that are open-source but the official versions of k that most people use "in anger" are commercial products with a limited free trial, and afaik there's no mature open-source versions of kdb+/q, which are kind of k's killer app. There are many implementations of APL but Dyalog is the clear leader and it's a closed-source commercial product with a personal/non-commercial free version. I wish this was less of a factor because it's so hard to get people interested in languages when the best versions aren't available to them, but it has gotten better in recent years.

Regarding tooling, you should go in with minimal expectations. Some of the tooling is quite good (particularly J and Dyalog APL, in my opinion) but it's heavily biased towards the specific type of iterative, interactive development that nearly all array programmers favor. Debuggers are sometimes present but usually not a primary tool. None of the major array languages have static typing. There are some array-adjacent languages like Futhark and Dex that do, but they're very different than the "Iversonian" array languages you asked about, and are also active research projects.

(Edit: Also worth mentioning that package managers and build systems are not common in the array world.)

There are many other differences that matter immensely to the array community but you won't have context for as a beginner, so I'm not going to go too deep into them, but if you're curious, https://github.com/codereport/array-language-comparisons has some comparison tables and example code written in a variety of languages. code_report/Conor's Youtube channel at https://www.youtube.com/@code_report/ is also an excellent place to get exposure to various array languages and concepts.

All that said, in my opinion the easiest languages to recommend to get started are BQN and J, depending on whether you want glyphs or not. If you're comfortable using a closed-source tool with restrictive licensing, Dyalog APL is also an excellent choice. Any of the three will show you both the joys and pains of array programming if you put time into learning it, and give you enough context to make an informed decision about going deeper or finding another array language more to your taste.