I like this general idea - lowering the curve, changing the manabase to allow good monocolor spells. It seems more viable.

To improve the mana (with fetches+duals+basics), you should probably either narrow down to 3 colors (XWr) or make blue a central color and run Xerox cantrips (UWbr). That's how other Legacy decks make functional manabases out of fetchlands + duals. Going 4 color without cantrips isn't really stable while Astrolabe and DRS are banned. Basically, I would think harder about which colors are most important to prioritize here to get good multicolor effects.

Some of the best multicolor removal is in BG (Assassin's Trophy, Abrupt Decay, Grist, the Hunger Tide). Unfortunately that would put you in GBWR (4 colors without blue), which is not really viable unless you base it on a lot of green mana fixing, so I see why you cut green.

Without that BG removal, I think you definitely want Prismatic Ending in there. Even though it doesn't trigger general, it's just too strong and flexible.

I guess your main color options are:
BWR (Mardu - discard, premium removal, KComm)
UWR (Jeskai - counters, premium removal, Meddling Mage)
UWrb (4color - cantrip fixing, best multicolor cards of each)

You could try what you have above (4 color without cantrips) but I think you'll run into mana issues.