Correct but GOS reverses alot of Google patches like always on voice requires kernel privalage it is disabled on GOS etc. But kernel level signature spoofing gives way for a malicious app to spoof as micro g and infect your device and you would never know because micro g requires the same thing to function it is making itself look like Google when it is not google. So using microg opens your device up to allot more ways for it to be compromised and also makes it harder to detect or notice once it is compromised. For me the security risk of kernel level spoofing is way to high to use on a production device used everyday. Also I trust neither Google or microg I only use Foss apps I don’t have Sandboxed play services installed at all I just don’t use Google anymore.
Then buy a newer one with longer support this will always be a issue since the support window is the same as Google. Once a manufacturer stops updating drivers and device firmware the said device can no longer effectively be secure because any exploit in the drivers or firmware will forever go unfixed compromisimg the devices security. Doesn’t matter what devices you buy this will always be the case it just depends on what your personal threat model is.