Strategy to free up slot for a 50 amp 240 volt circuit
MWBC's and double-stuff

The worst danger to double-stuffing breakers is encountering a multi-wire branch circuit.  In that case, double-stuffing can overload the shared neutral.  However, I don't see any markings on your wires, so it appears they are installed in cable, and all your single-pole circuits are installed with their own /2 cable.  If that's so, then you have two multi-wire branch circuits in the box, already on 2-pole breakers where they belong, and I trust you'll keep them either on 2-pole or the inner 2-pole breaker of a quadplex.  These are 


disposal --- dishwasher
kitchen receptacles #1 --- microwave oven &amp; oven hood


Alien breakers

You have a Siemens panel, but you have Eaton "BR" breakers in the panel (the ones you consider candidates for replacement). Those are alien breakers and should be removed immediately. They may seem to snap in, but if you compare them to how a proper Siemens breaker snaps in, they feel different; that difference is the breaker not engaging properly to the bus stabs, which could cause arcing under load. That will destroy the bus stab, and now you'll have an 18 space panel.  So those BR breakers disappear - today.   And the guy who installed them isn't invited back. 

What might've confused that guy is that Eaton does make a "CL" breaker line, specifically UL-classified to replace your Siemens Q115/Q120 breakers. However BR is not that line, note CL breakers are not classified for BR panels.  CL costs more and is much harder to find, so there's no earthly reason to go that way unless you have a high volume commercial account with an Eaton dealer (or want Eaton-only exotica like CLR "remote control" breakers).

This panel...

You are correct, AFCI breakers are not available in double-stuff. The most you could do is combine 2 circuits onto one AFCI, but it appears your existing AFCI breakers are heavily oversubscribed already, serving more than one room each.  Somebody really, really chintzed out on this panel, and is putting you to considerable inconvenience.  Well, I hope they really, really, really enjoyed that pizza (the one they bought with the savings on not getting a properly sized panel).

At your very first opportunity, you will want to install at least a 20 space subpanel, either here, or somewhere more convenient to your loads.   A proper house, especially one with 200A service, needs 40+ spaces.  There's always another load, and "I'm out of space, help" is a top question here. 

Best plan going forward: option 1

I see a couple options. The first one is simpler/cheaper, but the second one prevents future problems. 

Replace the three alien BR breakers with a Siemens QT type ("Q" Twin) Q2020 duplex breaker, and a Siemens Q115 one-pole.  That frees up one space, and gets you home for about $14. 

Option 2

You know the problem where someone replaces a multi-wire branch circuit 2-pole with a duplex, and overloads the neutral?  This option has a big advantage: it heads that off.  This overstuff problem isn't going to go away, and somebody's going to come along and want to compress those big 2-pole breakers.  So we're pre-emptively double-stuffing the MWBCs so the next guy can't screw them up.  

We'll use quadplex (Siemens calls them Triplex) breakers.  And by the way, MWBCs require common maintenance shutoff, so they must be on the inner 2-pole on a quadplex. The outer 2-pole won't do, even if they're handle-tied, because the handle-tie is too flimsy to assure common maintenance shutoff (and the labeling says so). 



We replace the 15A 2-pole MWBC (Disposal/Dishwasher) with with a Q21515CT quadplex.  (15A singles and 15A inner 2-pole).  We are stuck doing this because this is the only quadplex readily available with 15A in the center.  That is where the MWBC goes.  The two outer 15A's are independent 1-poles.  Your panel has two singleton 15A circuits -- "Furnace" (which must be a separate breaker) and "Garage/GFCI".  Those move to the outer terminals of this quadplex.  

Then, let's kill two birds with one stone.  Pre-emptively replace the "Kitchen#1/Micro-hood" MWBC 2-pole, in similar fashion, with a Q22020CT 20A-all-around breaker.  Then, likewise, move two 20A single circuits to the outer positions.  I would say go with what wires will reach.  This does one other useful thing -- frees up two real Siemens 20A breakers.  Those replace the alien BR breakers, which go on Craigslist unless you need them to fill empty holes. 

This scenario will cost you about $40 plus some 12 AWG pigtails, because Mr. Snippy didn't do you any favors by nipping off the wires to barely what's needed to reach the breakers now.  Future note: Every circuit should have its hot and neutral both long enough to reach every breaker space in the panel.  Neutral for GFCI or AFCI. 

This option will create some empty holes in your panel.  Slap any handy breaker in there, and don't connect it to anything, just put a sticker with a red X on the breaker. If it's a BR breaker, spray-paint the whole breaker red to deter people from using it.