Absent son

This is nothing to do with you. If your mother says she is ok with the level of contact why don’t you believe her? Stay out of it - as you say it only leads to arguments, so one has to ask why are you so intent on upsetting things?

To be fair with an 8 hour round trip I probably wouldn’t be visiting my mum as frequently as I would if I was half an hour away. That’s seems obvious?

Unlike others I don’t think the long drive is a big deal. I do however just think he and your mum are both comfortable with the fact that they are not as close as you and your Mum. Family’s are complexed, you have more of the work but you evidently have a better relationship, you win some you loose some.

His level of engagement sounds normal to me. I go home 3.5 hour drive 2-3 times a year and stay with my parents one day and my in-laws one day. My parents come up once or twice a year too. They are in their 70s. My brother sees them more regularly as he didn’t leave the small town we are from. I have a full time job children a DH and a life to lead too. I call the weekly.
Mostly you sound a bit bitter rather than wanting him to spend quality time you want relieved of the burden and want him to be there half the time. Your mum is happy they sound like they have a good relationship and all I would want from my child is calls and to know they are happy.

My brother only visits our elderly mum once or twice a year and it causes tension within the family.

Does it cause tension within the family or does it cause you to cause tension when you raise your concerns about it? Because you go on to say you are the only person who is upset about it so how does it cause tension?

You are being unreasonable. Your brother's relationship with your mum is nothing to do with you. He lives a long way away and comes occasionally. Your mother is happy with this. You need to mind your own business.

Unfortunately an 8 hour round trip is a lot, particularly if he has other commitments/pressures. That’s also presumably on him to live that distance, but it’s not surprising that’s the outcome.

He lives 4 hours away
He's in obviously close contact with her.
Absolutely non of your business. Your mum seems fine with it why aren't you?

You can't make him be more engaged. Unfortunately none is us know the reason, it may be valid or it may not be.

She claims that it doesn't bother her that he doesn't visit, and it's all ok because he texts and calls her.
So, when we discuss this, it inevitably gets heated, because I don't believe she's not bothered and I just want her to be honest with me.
So he lives a 4hr drive away, visits for a couple of days a couple of times a year, calls and texts to keep in regular contact with your mum the rest of the time, and your mum has told you she’s happy with this arrangement. What’s the issue?
Why do you think your mum is lying to you? Have you ever considered that she might be being honest and you’re starting arguments and causing her stress for no reason?

He also interferes from afar, and she doesn't always tell me.
For example, he arranged private medical appointments when she was already having nhs treatment and so on, and he also makes various suggestions.
That was kind of him. Was your mum grateful for the medical appointments, or for him trying to help if she didn’t end up using them?
What’s wrong with making suggestions?

The rest of the family say just don't bother asking anymore as it's just me who is upset about it.
If it’s only you that’s upset about it, and you’re causing friction by fixating on this/starting arguments about it, maybe you should listen to the rest of your family and drop it. This sounds like you’re the only person who has an issue with it, and as much as you’re annoyed with your brother, you might find everyone else thinks you’re the one causing problems if you carry on.