unfortunately there is an unhealthy amount of predatory game progression in it (i think it takes several years to be able to collect everything in it now thanks to the various forms of online dailies for shop gacha currencies) but outside of collectable aspects 100% Orange Juice is genuinely pretty comfy. I spent a solid year or so playing that occasionally in singleplayer before ever even touching online mode though i'm admittedly probably the only person ever to do so. It still supports its original 800x600 resolution and plays about as well as it ever did on low spec laptops and has plenty of charm to it
worth mentioning also that if you get into that (being an indie anime vg board game themed around other games from the developer), their other games are decently replayable in their own right, though they might be a little overwhelming...
The QP Shooting/Xmas Shooting games are vertical shmups where the key gimmick is that you can customise your shot types by picking three from a total of around 20-30 different formations you buy in the shop so there's a lot of replayability there. It consists of 3 (technically 4) games and 1 free expansion, though half of them are untranslated (you can find the last 2 in english on steam though). They're also probably Orange Juice's cutest games. Then there's the suguri series (comprised of Suguri and Sora), which notably is a series of horizontal shmups where you have a lifebar and can also customise your loadouts with more stuff you collect in runs, and there's also melee attacks. Those games let you progress 1 stage at a time so while I'd say they're not very easy, they are relatively accessible. Also they got some spinoffs called the Acceleration of Suguri series (X-Edition and 2 are on steam) and those are actually shmup fighting games and tons of fun. Pretty goofy stories in those too. Flying Red Barrel is a more traditional vertical shmup where the main feature is that you're just meant to manually spam rockets at enemies to increase your damage and point multiplier (well, it actually just makes enemies drop more coins you have to manually collect but same difference). It's definitely on the easier side as a result and also has 17 different endings depending on boss characters you show mercy on which also obviously gives it a lot of replayability, not to mention just trying to maximise your score gives it a lot too. Lastly there's also some extra titles with joke plots. 100% Orange Juice is arguably one of them, though you can also find their other anniversary game 200% Mixed Juice which has a pokemon-esque jrpg battle system with rock paper scissors mechanics on Steam too. It even has online, and a pretty hefty overhaul patch i contributed to. Their other ones are simple and free short titles usually released for april fools, including the gag title QP Kiss and the meta jrpg DEKU based off circle's composer and artist, both of which have fan translations as of this year. Their others include a terrible lucky star-themed action platformer called Gun Star, a red barrel parody called Flying Crimson Barrel (inspired by porco rosso), a fangame for an old ps1 jrpg dating sim called Mid Night Pleasure (the ps1 game was called Eternal Melody), and also an old puyo puyo clone called Fruit Juice (Kaju) 100%.