The specialist is right.
The guidance to move to continuous progesterone after either a certain period of time postmenopausal or by age 55 (not 50) is just that, guidance, not the law, and clinical judgement taking into account individual needs should always be used.
Sadly many GPs have neither the time, education or interest to go into this and also the NHS hasn't the resources to offer endometrial monitoring so they tend to just take the pragmatic but blunderbuss approach of overtreating the majority in order to protect those at highest risk.
It is sensible to get a private ultrasound yearly if you are deviating from these guidelines, in fact I believe this should be part of routine well woman care and I have one myself for ovarian screening.
If you are happy on your current regime and have neither irregular/unexpected bleeding nor a thickened endometrium on your scan, I would simply stick to your current regime given this is working well.
If your GP is being really rigid it would be better to order the increased amount of progesterone as if you are taking what they want, and hand back surplus to the pharmacy.
If you aren't comfortable going it alone you could have a consultation with a knowledgeable private specialist to back you up, as long as you had regular scans.
It is also worth knowing that micronised progesterone isn't your only option if tolerability is an issue, there are multiple alternative progestins that are often better tolerated due to them being modified to be effective at microgram or low milligram doses.