Running ser cable from 200amp main to 100 amp sub panel
Your support plans are fine; just make sure your feeder breaker is 90A or less
SER cable used for feeder or branch-circuit duty is treated exactly the same as a (rather fat) NM cable for support and installation-technique purposes, albeit without the 60°C working temperature limit on ampacity calculations:

(4) Installation Methods for Branch Circuits and Feeders.
(a) Interior Installations. In addition to the provisions of
this article, Type SE service-entrance cable used for interior
wiring shall comply with the installation requirements of Part II
of Article 334, excluding 334.80.

As a result, since the NEC only requires NM cables to be supported at a 4½' interval:

334.30 Securing and Supporting. Nonmetallic-sheathed cable
shall be supported and secured by staples; cable ties listed and
identified for securement and support; or straps, hangers, or
similar fittings designed and installed so as not to damage the
cable, at intervals not exceeding 1.4 m (4½ ft) and within
300 mm (12 in.) of every cable entry into enclosures such as
outlet boxes, junction boxes, cabinets, or fittings. Flat cables
shall not be stapled on edge.
Sections of cable protected from physical damage by raceway
shall not be required to be secured within the raceway.

Note, though, that while people may say that your 2-2-2-4 SER cable is good for 100A, that's only true when it's feeding an entire dwelling unit (house or aparment); for other applications, like yours, you're limited to the 90A ampacity of a 2AWG Al wire at 75°C by the NEC 110.14(C) termination temperature limits, unless you have an old main panel that isn't marked for 75°C usage, which then limits you to 80A instead.
GO BIG OR GO HOME
Assuming you haven't already acquired the subpanel, there's no reason to skimp on subpanel spaces.  While the heater may only require 4 or 6 slots for its own use, a 24-space or 30-space, 100A or 125A, main lug subpanel isn't that much more expensive than a mini-panel, and will provide valuable expansion room in the future.  This is especially true since the home of the new panel needs to provide a fridge-sized clearspace in front of it as per NEC 110.26(A) so that someone who has to work on the thing while &quot;live&quot; doesn't have to do a contortionist act to get to it.
Note that when you're hooking this panel up, since it's a subpanel (i.e. doesn't have a service disconnect inside it) powered by a feeder, you'll need to pull the green bonding screw out of its home in the panel, or simply not install it to begin with, and make sure that you install and use separate grounding bars to land your grounding wires in the panel if the panel doesn't come with them already fitted.
TORQUE ALL LUGS TO SPEC
Last but not least, you'll want to use an inch-pound torque wrench and/or torque screwdriver when tightening breaker and lug connections.  This is required by 110.14(D) in 2017 and newer editions of the NEC, and is a good idea anyway, lest your electrical system lose you the race!