Would you make the same argument for parts of continental Europe where borders are equally meaningless?
Take some of the borders between Belgium and the Netherlands which have also seen some of the same issues as the Welsh / English border has done during the pandemic.
The issues of complex border behaviour is not unique to the UK - far from it. Yet other countries manage just fine with having different rules on either side of "open" borders.
In terms of legal systems - that has literally no relevance here.
They are separate sovereign countries which do have legal control of their borders - it is not the same as internal borders within the UK. So legal systems absolutely is of relevance - Wales is one legal system with England and has no control over its borders. Scotland is a separate legal jurisdiction in terms of domestic law, but this does not include border controls - so not at all the same as Belgium and the Netherlands in legal terms (although there may well be many similar issues on the ground in those countries).
It would be much more logical for all of the mainland UK to have one set of rules - which could easily have been done by not devolving pandemic response (separate sovereign countries are not in this position). Northern Ireland is a bit different and it would make more sense for that to agree to a joint poliy with the republic.
What we have ended up with is Drakeford and (especially) Sturgeon using it for political purposes and having to be different for the sake of it. That helps nobody.