does it need to be new? because in polysynths it doesn't get simpler than the early Juno family and the polysix. I have both, they're dead fast and both have a great sound. The easiest to patch polysynths ever. Not particularly versatile but they each have a really distinct sound and a broad use case scenario. I have both and I never regret them. When I think "what do i want to play?" I usually turn on the polysix first actually. Its syrupy thick, plays great....g et one with the kiwi midi retrofit that's hopefully also been serviced recently though ;-) Junos may be too much money, that market is crazy.
New stuff that's easy? I like all of dave smith's synth on a chip ones, I have a mopho x4 but the prophet 08 and rev2 are also great, easy to patch but can go deep if you want. The whole fmaily is a colelction of riffs on a single chip synth engine where each has a different feature set, everything pre prophet6/ob6 is related like this except the prophet12 and the evolver which are kissing cousins and maybe worth considering too as their pallettes are very broad. They have a very specific sound that's like a punchy DCO thing but NOT a juno/jx sound... I also love the korg prologue. I just used it 2 days ago for a friend who wanted me to play on something. It is the best new keybed in my view though I've not tried the new rev4 prophets or the moog one. The main VCO based synth engine is very simple and limited but if you want some more texture you have the user oscillator, look that one up. But the synth itself excells at oberheim OBx type sounds and can do some pretty good vangellis sounds in the CS80 camp. It has some voice modes that alow you to create really deep bass patches in mono that's endlessly useful. Good enough digital effects if you want to play into the effect. The midi and analog clock implementation is glitchy on the arpeggiator though. Its very disappointing, but the arpeggiator is very primitive anyway and not worth using. If you really care about the arpeggiator and stuff go sequential.
Nothing else I own is easy. Modal's argon is endlessly cool and inspiring but its not easy to grok. I actually had to brush up my math to understand the manual. Its like a dx7 or k5 level of complexity but they gave you a lot of knobs. You probably want to avoid that.
Then of course there's the peak/summit which can be easy, layut's a poly minimoog on the front panel basically....its the most versatile thing out there if you deep dive. Again I've discovered some little firmware glitches in the summit but its really tweaky little stuff like the prologue arpeggiator, I can't even remember what they are. i think peak is better worked out. It sounds great in a very distinctly novation way. It has a lot of sound design potential if you want it but you can also just make some regular old patches that sound good. You read my review. I find I use the summit very little and it hasn't paid for itself. Its great bt can be a major time suck as you get lost in the endless possibilities... even the simplest task can turn into a ridiculous sound experiment, not because tis hard but because its actually pretty intuitive even the menus. Its not conducive to getting stuff done quickly. I found the peak to be almost as versatile and a lot less distracting.
I really think with your parameters in mind the top choices at a reasonable price today are peak, prologue and the rev 2... now the prologue is keyboard only and the peak is no keyboard unless you double up to the summit and its expensive then, and the rev2 you lose some knob per function in a module.
if money's no object by a rev4 prophet 5 or 10 and have done with it. I want a 10 so bad I can taste it but wow, I would have to sell my summit and then dump more money in!