I am switching out a 8 gauge aluminum wire on a 30 amp 2 pole breaker for my dryer less than 25. Can I use 10/2 copper? '
The #1 threat to your family is the 3-wire connections to range and dryer.
This wiring method was allowed due to intense lobbying by the appliance industry.  But what those connections do is BOOTLEG ground off the neutral wire. It was always illegal everywhere else, but this was delayed until 1996 for dryers and ranges due to their lobbying.
The problem with bootlegging ground off neutral is if the neutral wire gets loose anywhere, even just a socket pin, then it becomes a 100% certainty that the chassis of the appliance will be energized at 120V. Touch it and anything grounded (like the washer, or a sink, or another 3-prong appliance) and BLAMMO.
You must admit this is a &quot;freshman&quot; attempt at improving safety, which is under-supported by good research.  You're on the right road though.
Oh... and next time, deal with a real electrical supply house.  Home Depot is a terrible place to buy almost anything.  Their selection is limited, their stocking is terrible, and their prices are ridiculous. (they once bothered to undercut other businesses, but now that you're convinced they're the best deal in town, they stopped undercutting and are just rooking you. CostCo is doing the same thing BTW.) As things were, since they were out of 10/3 it would've been wiser to go to 8/3 (you're always allowed to upsize).
Anyway, replace both the dryer's cord and the receptacle with NEMA 14-30 type, and also remove the bonding strap on the dryer, which is responsible for bootlegging ground. Very important, that!
The heavy aluminum wire is fine, but...
Obviously you've found some articles on the Internet that rips up aluminum wire.  There's validity to those, as regards small 15-20A branch circuits such as to lights and receptacles, but it wasn't aluminum's fault.  Aluminum lugs happily take any wire (that's why your panel uses them) but copper lugs do not like aluminum wire.  In the 1960s in response to a copper shortage, they hastily approved for alumiunum use a bunch of outlets and switches previously designed for only copper.  Those copper lugs refused to play well with the aluminum wire, and there were serious problems at the terminations.  Also the installers were poorly educated on use of aluminum wire.
Mind  you, heavy aluminum feeders have never been a problem.
Since then, every single thing about aluminum has been changed.  A new wire alloy was made specifically for branch circuit wiring, called AA-8000. (the old AA-1350 alloy was straight up power-line aluminum, yes, the power lines are aluminum).  The approvals were canceled and a new standard - CO-ALR (R for Revised) - now applies to switches and receps.
Now, your #8 feeders to dryer and water heater were wobblers: at #8 they are in the gray zone between the worrisome &quot;#10-12 small branch circuit wiring&quot; and &quot;#6 and larger reliable heavy feeder&quot;.  I would treat them as feeder; make sure they are terminated properly, then don't worry about it.
Each appliance
For the water heater, my recommendation is to double-check the connections to the Al wire at both ends - make sure they are on Al-Cu breakers (which were not affected by the CO-ALR standard change)... and on the other end, go into a modern approved terminal that is CO-ALR.  If it's not, no big deal -- just pigtail it with copper, then use an approved aluminum splice.  For mortals, that means an Alumiconn (#14-10), MAC Block (#14-#6), or Polaris splice (#4 and larger).  As long as the connections are clean and torqued to spec, I wouldn't worry about it.
Use a torque wrench to tighten the screws.  Very important. Most splice failures trace back to improper torque, and even electricians get torques right only 25% of the time (and that presumes +/-20% tolerance, which is pure luck, so basically nobody has a calibrated arm.)
Of course with the water heater, you now have that 10/2 cable you improperly used for the dryer.  That has to come out.  If you take it up carefully, you can reuse it for the water heater, as 10/2 is legal there (re-mark the white wire to be a hot wire; water heaters don't need neutral).
For the dryer and range, the deal-killer is the 3-wire connection.  So we definitely will replace the dryer's cord and receptacle with a 4-prong NEMA 14-30 type, and rewire the range as appropriate. This includes removing the neutral-ground bootleg jumper from both range and dryer.  What wire do we use?  It's legal to retrofit a ground wire, so you could continue the old aluminum 3-wire in service (as I advise on the water heater), IF:

The cable was a legal type at the time it was installed, meaning either a) an insulated white neutral, or b) SE cable (neutral webbed around the hots).  AND
The socket is CO-ALR rated, or, you pigtail with MAC Blocks.

However given that you already know the drill, and the cost of Cu 10/3 is not extreme, you might just replace it with that.  Remove the 10/2 carefully and use it for the water heater.
You cannot retrofit a neutral or second hot wire, nor can you retrofit on stuff you just installed... so the 10/2 has to go.