I like that you have self awareness about these things. I think the crucial thing to realize is that "senior" means very different things to different organiza

I like that you have self awareness about these things. I think the crucial thing to realize is that "senior" means very different things to different organizations and people. If their idea of senior is more in line with other places idea of mid-level then taking the position might be really good. If they have made an offer then it shows they are willing to pay you that amount. If they have assessed your skills and their needs correctly I wouldn't be opening a conversation by offering to work for less. If there is a big gap between what you will be expected to do and what you can do then this is a concern. Address this potential gap in skills, do you have other people you can learn from? Access to external help if need be?