I'm not entirely sure how long I've been playing, but I've been playing since some of the earlier updates, I think since before the swamp was added? It was, as you might expect, a while ago. I've hit the max level, though. As with a lot of porn games, it does fall off a bit in the later parts, in that the intermixing of the gameplay elements of relationship building and exploration/combat sorta gives way to "just focus on one thing" when you're wrapping up most of the characters' stories, and you've collected most/all of the unique treasures. So, generally less of the "multitasking" sort of aspect where you have lots of different options of things to do.
I believe the shop restocking isn't tied to level, but to what things you've purchased. Once you purchase something, a new thing will take up the spot after a day. Something like that. Again, I'm not sure. It's been a while since I've had anything much to buy. The outfits do keep restocking until pretty late, though. You'll have the other shops bought out far before the outfit shop, simply due to the material components being tough to get. Although, a lot of the shop outfits are entirely outclassed by some of the outfits tied to upping affection with the other characters (eg. the PJs set), so don't stress it too much.
I agree, the bars aren't very useful for determining relationship status. Although, if there's still bars left to fill, that's a sure note that there's more to do.
Most of the battleable characters have a reward from beating them once, and I'm 99.9% sure that none of them lock you out permanently if you defeat them. That Dark Bomb spell is quite useful, and once you defeat the Demoness, she'll come back a day or two later like nothing happened. And I think you can always defeat her later to get it. Same goes for other characters. Defeat 'em once when you have the power for it, then don't worry about it. (I think there might be one exception way later on for a certain unique snake lady in some ruins. But I haven't gotten there yet, so I don't know) In regards to the male characters, none of them have any romantic/sex scenes, so no worries there. But, you may have to interact with them a bit to progress in the main quest (and Evie's, I think? Again, it's been a while).
In terms of the wallet... Basically, all the "X stone" items (Ruby Stone, Jade Stone, Topaz Stone, etc. I think that's what the items are called, at least?) can be fused with the wallet in a certain order to upgrade it. I think you only get them in the order they're Fused in, so don't worry about doing it in the "wrong order". I don't recall where they're found specifically, but I'd recommend regularly checking the fortune-teller and asking about Treasure each day, and possibly taking notes about where stuff is. That way, you can use porcess-of-elimination to see where the wallet stones are. The elemental stones are all key items; once you have all 6, they're used to unlock a door, I believe in the desert? Not at all related to the wallet, and they're not Fuse items. I'm not sure what you mean by "accessory gems", since I definitely don't recall any gems used for accessory upgrades (besides those for the wallet), so you might try fusing those to the wallet. Another Fusing tip for you: I'm pretty sure that Accessories are the only thing you can fuse "metals" (silver, gold, mithril) to, and nothing but "metals" can get fused to accessories. And the bulk of Fusion recipes are Accessories.
I think I've confused Skill Points and Ability Points before, so here's something to note: Ability points, what you use to level up your spells, are what I called skill points before. How you get more actual Skill Points is via learning more Skills, and there's at least one Skill Point Book somewhere. I don't think it correlates one-to-one, but the more Skills you learn, the more points you get. However, as you progress, you'll get further and further from being able to equip every single skill you get, and you'll just have to live with that.
Generally, you shouldn't sell stuff, except for ingredients. The accessories may have more upgrades in future updates with additional tiers of material (there's 4 material tiers at the moment). And even without that, you might want to equip already-learned accessories to get their buffs without spending Skill Points. Although, I suppose you can sell them, if you really need to. You'll probably need to buy them back later, though.
Same goes for "weird" or other quest-related items, like the Dildo. Later events may make use of it in this version or later updates of the game. The dildo in particular is useful in gaining the affections of a certain elf later on.