I've known of Zoomer for quite some time and he seems to be doing nothing but harm. When I spoke to him he basically admitted to having next to no theological knowledge, yet still thinks himself fit to teach (mainly using YouTube and Instagram). There are many serious issues with his theology, he is an ecumenist, an evolutionist, does not think people should leave churches over "female pastors", and pushes new converts to join denominations such as the PCUSA with the goal of reclaiming their buildings. He tends to misrepresent Presbyterianism too which has not been helpful in any way, and has been very critical of denominations such as the OPC.
The basis for this "Reconquista" idea is really just shallow appeals to tradition and aesthetics - there is no real depth or legitimate Scriptural reason behind it. Due to this shallow basis, many of his followers 'converted' to Eastern Orthodoxy when Zoomer lost to Jay Dyer in a debate - he of course sees no real issue with this. I think the whole matter is very foolish, all that this is accomplishing is boosting the numbers of apostate churches at the expense of new converts. I think 2 Chronicles 19:2 is very fitting to this whole situation.
There is also a fundamental flaw behind this idea. He thinks mainline churches and institutions should be reclaimed because they influence the culture. If this were true then that would be a legitimate reason to try and do so, but the reality is that we live in a post-Christian society where the mainline churches have no cultural influence whatsoever. Mainline denominations do not steer the culture, rather they are steered by the culture - that's the sole reason why they have people and money. His whole idea is very similar to Neo-Calvinism, and yet his is far less thought out.