Can I use a digital signature for the Declaration form for my Thai Tourist Visa if I don't have a printer or scanner?

You don't need either a printer or a scanner. Either fill out the TDAC online on the official government website tdac.immigration.go.th or using the official THIM app. Once you submit it, you receive a confirmation email but it's never asked for unless they can't match your submission with your passport number when you give the immigration officer your passport to enter.

Yes

Yes have digital signature on a few documents. Immigration accepted, no problems just had to counter sign in front of the IO as verification

just apply online and it will be emailed to you, show the electronic email version at immigration check in is fine i’ve done it. dozen times

Absolutely yes