1a. Hey, always remember: don't stress over how you differentiate the tiers from each other, as the dev process goes along you find both new and improved ways of doing that organically.
1b. As a somewhat of a person with some knowledge on the dev side of things (Unity and UE in my past before 2020 shenanigans) two tips that make You stand out as a dev: If feasible, give the customers the ability to name their saves to at least 15, preferably to 24 character places In aDdItIoN to the timestamps given by any program You use and if You or anybody else thinks something should be a toggleable thing in preferences, if You find it is easy, or even mediocre to do on Your skill set, (please) make it be an item in the preferences, or at least make a feasibility study on the integration.

2. Discord integration can be tricky especially if one doesn't set clean boundaries on what is permissible so that they don't get shut down because of one jerk, but, a big BUT(T), Discord can help facilitate translation to other languages that one doesn't even think of being a possibility (and refining the different language scripts (I personally take credit on making such a change happen in Lust Academy 2, as MC had a problem differentiating between a binary question and a question of scale and now there is none)) and in obvious-to-user-not-obvious-to-dev bug crunching. If You don't find the chirpy Discord fairy in You, I'd advise part-time hiring or some such arrangement for a publicist for all Your communication needs in not so distant future so that They can generate (and to an extent moderate) content for all to see (There's a Patreon goal to set after Your own basic needs are met via the self-same target system).

3. Custom art is fine as long as it doesn't detract from the process of making the game. In my personal opinnion there is nothing worse than a dev milking any platform for money whilst making close to nothing to finish the original product (there is a reason why I follow You on Patreon, but haven't crossed the line to actually giving any money (some Ren'Py devs can be scummy as *insert cultural place/form for those not deemed worthy people to go after death* itself)).

P.S. You could add to every tier above the lowest a line: Includes and/or improves the benefits of the previous tier.

P.P.S. Always, always, always, always try to be the one who creates their own Discord server and then relegates monitoring it to somebody else before somebody hijacks it for nefarious purposes. I just created and nuked Discord server named August Esoterica in my native language, the next, or at this point the previous person may not be as sympathetic to the idea of deleting or giving over their channel if money is not exchanged. I noticed someone already had that server name taken in English so I hope it was You.