There are three basic types of addresses in common use today:

Pay to Pubkey Hash - aka P2PKH - aka "Legacy"... these start with a "1"
Pay to Script Hash - aka P2SH... these start with a "3" and there is a special subtype of P2SH called "P2SH-P2WPKH"... this is known as "Nested SegWit".
Pay to Witness Pubkey Hash - aka P2WPKH - aka "Native Segwit"... these start with "bc1"

Bitcoin transaction fees are calculated based on the data size of the transaction being broadcast (and since the implementation of SegWit... this "data size" is now calculated as a "weight"). The fee has nothing to do with the amount of BTC being sent.

Now, given that Nested SegWit (P2SH-P2WPKH) and Native Segwit (P2WPKH) are able to construct transactions that have a lower "weight", they are able to be sent with a lower total fee that a "Legacy" transaction.

However, your idea of a "fixed" fee is a bad idea... because transaction weight is variable and not necessarily guaranteed to be the same from one transaction to another. So, you might find that while you use the same total fee, the actual "fee rate" (which is what miners use to prioritise transactions for inclusion in a block) could vary from one transaction to another by a significant amount.

for instance... say you wanted a fixed fee of 0.00010000 sats... if your transaction was 250/vbytes... your effective fee rate would be 40 sats/vbyte.... however, if your transaction had a number of inputs and was 1000 vbytes, your effective fee rate would only be 10 sats/vbyte and prioritised much much lower.