Not an easy situation and one I'm sensitive to as well - but from the other side. I'm 56 but just started playing out a few years ago in a couple of basement jams and then finally joined a gigging band last summer. Every one of the people I've played with has decades more experience than me and two have been playing longer than I've been alive. It can be intimidating, but it does motivate me to work at it.
Recordings can be a big help. Sometimes I cringe when I hear them, especially considering that I'm the gigging band's primary backing vocalist (two others will add some backing vocals when we need more than one voice but, if there are backing vocals, I'm in on them) and also have my share of lead vocals. Recordings are as objective an assessment as you can have - and they can sometimes surprise you and alter your original impression (for better or worse!

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My band knows that I'm relatively new at this and is really supportive, but our LS/BL doesn't hesitate to offer suggestions to me, especially with vocals. But the entire band does a good job focusing on how we sound as a whole and offers suggestions in a constructive, rather than complaining or accusing, manner. Maybe I'm lucky that we're all adults and act like it, but I really appreciate that about us. And, just to be clear, I am absolutely NOT saying you or your bandmates are acting childishly. It does sound like the band handles it well. Burt there's no shortage of posts on TB complaining about the immature and childish behavior of band members.
To be honest, I do sometimes simplify my basslines when I have vocals to do. I've told that to our LS/BL and she agreed that it's usually more important to get the vocals rights since more people will notice that than bass (that's possibly equally true for drums?). Then, once I have the vocals down and a solid bass line, I'll keep working at home to get as close as possible to the original bassline. I did say "usually more important" because there are times when there's a signature bassline that needs to be right so I work even harder on those. Are there songs where the drummer may be able to start out playing more simply at first to get the vocals right but continue working on the song to get closer to playing the original drum part?
Is the band also open to putting some songs on the backburner for a while to focus on songs that you're doing well or look for songs that you can do well now? We put quite a few on the backburner when we first started rehearsing last summer and, at least pre-COVID, would regularly decide to re-visit them. We'd occasionally decide to practice one or a few at home for a while and then bring into a rehearsal in a few weeks. We'd play them a couple of times, see if there are remaining issues, then then decide what to do with them. Sometimes we have them down well enough to add to the rotation. And then some get backburnered again and wait to be brought up again later.
I understand being frustrated and feeling held back by someone who's not at your level and see a lot of posts on TB recommending that that person be let go right away. But I also see a lot of posts saying that the best way to improve is to always play with better than you. Those two "truisms" have a tense co-existence.

Since the lead singer is good friends with the drummer, maybe talk to the singer to see if she has any suggestions, such as being specific about some things to work on each week (or whatever the right interval is). In other words, simply saying "These 15 songs stink and need to be fixed now" probably won't accomplish anything. But, as a band, come up with some specific things to work on for the next rehearsal, pointing out things that everyone could work on. For the drummer, offer some specific advice on specific songs, do some quick reviews of them while still at the rehearsal (i.e. play through them to point things out, good and bad), maybe suggest some YouTube videos that break down the drum parts for songs you're working on, etc. If the drummer is interested in improving, you'll see it. If not, then you'll see that too. But at least you'll get a good idea of the kind of musician you're dealing with and can make a better decision about what to do next.
Sorry for the long post, but as someone who's much closer to your drummer's situation than yours, I tried to offer things that I've seen in my own situations and also seem to be effective for me personally and for the band as a whole.