I agree about the pre-mined aspect being problematic. It is a bit of a pity that the current incentive structure in the "altcoin world" favours these premined solutions. Many of the Bitcoin sidechain projects seem to come either from the Ethereum world or from the Ordinals/Runes "scene", where premines are seen as completely normal.

But if the 2-way-peg solution of one of these pre-mined "Layer2's" works, then it would also be possible probably to fork that project and use a hybrid PoW / PoS token without any premine to sustain the sidechain. The PoS is probably not even necessary for block validation on the sidechain (because that could be detrimental for the "censorship resistance" aspect), only for the federation members - so there is an incentive structure to not misbehave, including slashing wrong peg-outs with an amount higher than the pegged-out coins. For good behaviour they could also receive a reward separated from block validation, similar to "masternode" systems currently.

It's possible a Bitcoin-pegged token (with 2-way peg to the mainchain) could also exist on an existing altcoin with a strong own consensus, like Litecoin. The protocol for the pegged token would however require probably turing completeness, or the rules to be be hard-coded.

Other solutions like Drivechain (BIP 300) do not require PoS at all, but they require an upgrade of the Bitcoin protocol to add opcodes.