I looked at a few options like this, but listed eveything that I'd need/want to add and the price escalated quite quickly. Once you add in insulation, interior panelling, a second battery, lights, and wildernest tent the total price will be quite high. Compare that to a used Skamper pop-up or similar at around $1500 with minimal work/money needed to make it comfortable if the stove, battery, furnace etc already works... plus if you pick up a complete camper you can head out the very next weekend.
Just be honest with yourself and write out a budget including all of your wants and needs. Selling your topper would go a long way to paying for a used camper too...
I understand wanting to get away from the topper. Just not enough room to be comfortable when the weather is poor. A roof top tent combined with the topper is great for having space for gear/dog and a separate sleeping area, but there's no "living space" and you can't heat the roof top tent. Then you add an awning for a couple hundy for some dry area when it rains... and that option gets expensive quickly too.