2nd tier adds almost nothing for footprint, or for setup really. Its one more power cable, one more signal cable, one midi cable plus the tier maybe.
I have a 3 tier setup, with an RD2000 on the bottom, an FA07 in the middle, then a legend EXP organ module, the Peak, and the Argon 8. I also play guitar and carry an AFX 4U rack, a 2x12 cab, pedalboard and sundries - and 2 guitars. I also carry 2 x PA tops (for keys backline, or monitoring. I have my setup AFTER loading (which obviously changes) to about 20 minutes, teardown about the same. My stand has to part assemble (the top 2 tiers need screwing out/in to transport).
I probably woundnt look at the Argon8 at present. Its wavetable - so not the easiest to learn, and currently you cant save custom arp patterns to patches (its coming in the next FW). It does have "animation" lanes in its sequencer (meaning you can automate filter sweeps and the like) but so doesn the Minilogue XD. Even though I picked an Argon over the Minilogue - I did so because a. I have a peak already so a lot of crossover - b. I didnt quite have the length of space available the minilogue needs. The Argon is shorter. all in all the Argon suits my needs better - I feel the Minilogue would suit yours better.
Minilogue will get you there at the cheapest price. The Peak is 8 voice v the Minilogues 4 voices (but unless your playing 4 note chords AND a lead line 4 would be enough - probably) and has a better modulation matrix - but it doesnt have the programmable arp (just standard up, down, up/down and random) or sequencer. I think at twice the price of the minilogue XD it will be missing things you really want/need - while adding a lot you probably wont use.
The Juno DS - I "probably" wouldnt look at either to be honest. You have a great key action on the RD. Youd be better looking at something like Rolands FA07 ( a full blown VA synth section) or ModX7 IF you want a full size workstation type board. The DS is cheaper though - so if you did want to add another full sized board but didnt want to spend as much as the FAs/ModX then its a valid option.
Personally I think youd be better with a synth though (a small one - dont really need a full sized keyboard - and you can always drive it over MIDI from the RD is you do). There smaller - so you could get a couple on your top tie down the line. For instance you could add a Keystep AND a Minilogue XD (for less than a peak) and both would fit on the same 2nd tier. There are mono synths that would do the Hungry sound nicely, but you couldnt have nice synth pads and the like due to one note at a time.
Not that you NEED that right now - but you could. OR you could add a decent Hammond module (Like a Ferrofish or a Legend EXP) and a Minilogue - which will give a lot better organ sounds, PLUS a full 4 Voice analogue synth all on that one "2nd" tier.
A controller like the keystep is nice - but unless your adding a synth module with no arp/sequencer onboard, you NEED to have the core sounds you want/need already in you RD800. I couldnt do that with my RD2000 to be honest, but it does depend what your doing. If the RD800 makes all the noises you need - sure look at the keystep/pro - but be SURE your Ok with the sounds on your RD.
Lots of options - but let your budget and need drive this, NOT what you CAN have.