I think you need to sit down with your DH and really spell out what you’re worried about. The bottom line is this - the current situation works well BECAUSE of the distance. The physical distance prevents the aspects of her relationship with DH which would be a problem, becoming a problem. The physical distance allows you BOTH to maintain the relationship in a way which gratifies and satisfies her and this in turn means that you can successfully focus on the positives. Result - you all get on ok.
It isn’t just ‘moving closer’ … it’s ‘upending everything’ - and he needs to be made to really understand that a very likely outcome is none of you getting on very well any more at all.
You can easily spell all this out. So, she’s needy and likes things done for her and likes to see your DH as a big avenue of support. At a distance this works fine. Support can be there for the big things BECAUSE she is unable to expect to take up your physical time and effort with small things or to demand very frequent visits, because of the distance. You can easily overlook the elements of her personality you don’t gell with BECAUSE she isn’t there more frequently. Similarly, you can devote quality time to her for these visits as it doesn’t take up too much time. Result is that she feels cherished and all is ok.
But move closer and the potential for conflict and bad feeling is suddenly very large. She will want more time. But it’s not there - you are both busy, you have young children. She won’t get the attention simply increased to fill the time she now has to focus on DH. If he does increase the time, he does it at a cost to your family, which you won’t accept. She will feel let down. She’ll likely blame you. You don’t like her enough to want to increase time spent with her, which is fair. But that does not mean that your DH can pick up that baton as his first, and chosen, duty is to your own family life and kids.
The first red flag here too is her only discussing it with him. That should flag up to you both very clearly what the first issue is going to be. She isn’t seeing ‘your family’ as the people she’s choosing to move closer to - she’s just seeing him, her son - an autonomous person. That really should have been nipped in the bud from the start - HE should have been the one to push back saying WE WE WE in return - mum you realise WE have pretty busy lives, you are likely to not see US a lot more than you do now? That WE won’t be able to drive you around and get you to the dentist and doctor that’s a lot further away? Tell him to ask her, have you discussed this with Carebear? If not why not? I bet she’ll be completely astonished that there’s any need to, OR (worse still) get huffy that she’s not about to start ‘asking permission’ to make plans for what she wants from ‘her own son’s’ relationship with her. Red flag!!!
He’s being very naive here. She’s going to expect to be able to call on him - to do inconsequential things, to just spend time, to pander a bit. You will be saying no, not to him spending time- but to being able to carve out that time by pushing his family responsibilities onto you. There will be conflict and the end result will be - her blaming you, him being stressed, you disliking her (especially as you don’t have a lot of goodwill to fall back on). And she won’t be happier!
It seems, ultimately, quite unfair of him if he’s not spelling this out to her, because quite rightly, if he’s making noncommittal noises she will get a shock when he’s not able to give her a lot more time. And it will be too late by then as she will have moved! The amenities thing is also really important - she’s going to be a lot more out on a limb by the sound of it.
I would stick to my guns, say if she moves I predict a falling out before very long, a less happy marriage and a much less happy MIL, so talk to your mother and warn her that she currently has as much time as we can realistically give.