I'm really glad that you're planning to do a kickstarter for this, as I would totally back a kickstarter for this and will also keep an eye out on Steam so that I can wishlist The Radiants on there.
As a kickstarter backer, my preference is to back for digital rewards over physical rewards due to the fact that I am in the UK and kickstarter does not collect vat which means that I will generally get stung with quite a large bill, however I know that some backers do like to have physical rewards (maybe anything physical could be a stretch goal).
I love things like game guides and also like to back for exclusives like access to game betas if I am really into the game.
I find it fun when kickstarter campaigns have a discord as it can be a nice way in order to generate a buzz during the campaign and have backed campaigns where they give kickstarter backers a special role as well as campaigns where access to special discord channnels is a pledge reward.
I personally don't mind if games are not on consoles as I don't own a switch or any other console, but I know some backers really like that and that some kickstarter campaigns have things like that as a stretch goal.
I also am not personally a big fan of voice acting, although others really like it and I have backed campaigns where voice acting (either full or partial) is set as a stretch goal. Translation of the game into other languages is also something that I've seen set as a stretch goal sometimes.
Some kickstarter campaigns I have backed have a community goal thing - ie, sharing the game via social media (eg facebook or twitter) with enough shares unlocking something in game and whilst it doesn't work for me personally as I'm not really on social media, it's a nice way to create a buzz over the campaign.
For me personally I like to see things such as alternative poses or outfits, as well as side stories, prologues or epilogues and extra cgs. I also like stretch goals that add in extra lore eg. game encyclopedias and mini games as long as they are a fun side thing
I also like kickstarter campaigns where you can add things to your pledge eg. if you back for a certain pledge level and want to add on an extra game key or game guide etc.
One of the key things as a kickstarter backer for me is communication - being kept updated, via kickstarter, is a big thing for me. I am more likely to back at a higher tier if I can see that the project creator is updating regularly during the campaign - it doesn't have to be constant, but more than just an initial single post - although that's just me personally.
I do also think that project creators should keep backers updated regularly once the campaign is over - again it doesn't have to be constant - but I do think it should be at least monthly on kickstarter even if the game dev is also updating elsewhere (eg on social media) and have backed projects where communication is really good and have also backed projects (and really regretted it) where communication has been terrible.
Sorry this is really ramble-ly, I can't advise anything on the dev side or the running a kickstarter side but I've tossed in a mixture of things that I personally have liked as a kickstarter backer as well as things that other people have seemed to like so I hope some of the above is useful.
Also I forgot to mention (but am now editing in) that when thinking about stretch goals that it might be good to have a cut off point. I've backed some kickstarters in the past that have done really, really well but were perhaps a little overly ambitious with their stretch goals which in turn led to huge development creep - although again I am not a dev and wouldn't know where to start so it isn't really something I could really advise on.