IMO Sipeed Tang is the product most like Arduino for FPGAs: boards ready to program with a USB cable and a range of plug-in-and-go peripheral boards available. The Gowin IDE is free but not open, while the yosys+nextpnr open toolchain claims full support for the FPGAs on the Tang boards.
LiteX makes speccing out an FPGA core much more approachable for someone from a software development background, too, so LiteX plus a Tang Nano 20k and you can do all sorts of awesome things without too much trouble.
Citation needed. The GoWin FPGAs on these boards are still very much a work in progress. YRabbit is doing most of the reverse engineering on them, it seems, with the DSP blocks only implemented a couple weeks ago.
Project Apicula says "currently supported boards are ... [list of mostly Tang boards]" without qualification.
I wouldn't be surprised to discover that it actually is qualified support, though. Do you think the actual support is low enough that one should avoid those boards for now?
Whether there is enough for you right now depends on your needs, plus the knowledge that the support will continue to mature as time goes on. A good indicator of what you can do with the open source tools right now is checking out this link [0] at Lushaylabs. It has a fantastic series of practical hands on and useful articles like making your own CPU and driving HUB75 panels. Plus "This series uses a completely open source toolchain which makes development open / fast and lightweight". There is also /r/GowinFPGA on Reddit.