How can I best balance current loads among my existing kitchen circuits?
Bad news: putting the fridge on the range hood circuit is a no-go
Your first problem is that you can't, by Code, put a kitchen refrigerator on a general lighting circuit.  This is a function of the way NEC 210.52(B)(1) and its exceptions are worded:

(B) Small Appliances.
(1) Receptacle Outlets Served. In the kitchen, pantry, breakfast room, dining room, or similar area of a dwelling unit, the two or more 20-ampere small-appliance branch circuits required by 210.11(C)(1) shall serve all wall and floor receptacle outlets covered by 210.52(A), all countertop outlets covered by 210.52(C), and receptacle outlets for refrigeration equipment.
Exception No. 1: In addition to the required receptacles specified by
210.52, switched receptacles supplied from a general-purpose branch
circuit as defined in 210.70(A)(1), Exception No. 1, shall be permitted.
Exception No. 2: In addition to the required receptacles specified by
210.52, a receptacle outlet to serve a specific appliance shall be permit‐
ted to be supplied from an individual branch circuit rated 15 amperes
or greater.

As a result of this, you'll have to either put the fridge ahead of the GFCI on a small appliance circuit, or pull a new homerun from your panel.
Worse news: you can't put a built-in on a small appliance circuit
Furthermore, putting a built-in appliance other than a gas range, cooktop, or oven on a small-appliance branch circuit violates NEC 210.52(B)(2):

(2) No Other Outlets. The two or more small-appliance
branch circuits specified in 210.52(B)(1) shall have no other
outlets.
Exception No. 1: A receptacle installed solely for the electrical supply to
and support of an electric clock in any of the rooms specified in
210.52(B)(1).
Exception No. 2: Receptacles installed to provide power for supplemental equipment and lighting on gas-fired ranges, ovens, or counter-mounted cooking units.

And the kicker: you can't put your chosen microwave on the range hood circuit
Since you're using a built-in microwave, you can't put it on the range hood circuit either, as it'd exceed the 50% limit on fastened-in-place appliances imposed by NEC 210.23(A)(2):

(2) Utilization Equipment Fastened in Place. The total rating
of utilization equipment fastened in place, other than luminaires, shall not exceed 50 percent of the branch-circuit ampere rating where lighting units, cord-and-plug-connected utilization equipment not fastened in place, or both, are also supplied.

Note that if the lights weren't on this circuit, then you could put the microwave and the range hood together on it.  Furthermore, if you used a countertop-type microwave in a cabinet &quot;nook&quot; instead of a built-in appliance, you'd then be able to put the microwave receptacle on the range hood circuit, as well.  But, since neither of those options look to be in the cards, you're stuck pulling a new homerun from your panel.
As to that new homerun...
For this new homerun, I'd pull a 14/2/2 or 12/2/2 (two circuit) cable in order to provide dedicated branch circuits for both the refrigerator and the microwave.  You'll need AFCI protection on both circuits as well, although not GFCI since neither of them are feeding kitchen counter receptacles.