But actually I think fixing city state placement (on all map types) is a much higher priority. It's all too often the case that the city state is one or two hexes away from where it should have been located.
What makes you say this? From my experience with map scripts, I'd say City State placement is working as intended. When generating the map, it first breaks everything down into regions and subregions, categorizes them (river/desert/ocean/etc.), then determines the viability of each by seeing what Resources, Bonus Tiles, and Natural Wonders randomly spawned in each. From here it places players first, and adds a few extra tiles to the start locations if needed (A second Luxury Resource for example). Then, it places City States; improving them with Strategics and Bonus tiles if needed (but not a Luxury).
@OP, I don't think you really understood what the Ring Map Script did, which is understandable, as the Tool Tips can only infer so much information. Also, it states that you'll be placed on your own "subcontinent"; all the subcontinents are connected by isthmuses with a center continent specifically for City States. Rings is generally a bad map type for any Leader relying on or benefiting from city states. And the larger you make the Ring Map, the worse the effect is.
Venice would definitely benefit from any of the water maps (Small Continents, Large Islands, and Archipelago) with the latter two possibly being the best for it's UU as the Great Galleas is limited to coastal water and it's not uncommon for huge island strings to be connected via coastal water on them. That being said, Venice can do well on almost any map if played to the best of their UA and UUs capabilities. You could even completely forgo puppeting City States with MoVs and still do quite well between purchasing and conquest.