I really recommend DO for what you need.
I know it sounds complicated but here's what you should do (it's really NOT complicated at all).
2 tools
1. Docker 2. Nginx
Most likely everything can be on a single instance unless you have customers with lots of traffic.
Docker to host the customer code with mounting to `/customer-name` etc... so you have a persistent storage.
If you worry about losing data, you can rsync everything to another server and hotswap when something fails.
Nginx should be the router/balancer so `client1.com` will go to the docker host it should go to.
I work this way with all of my blogs and open source projects and it's really a breeze to manage.
VM on GCP is closer to Google cloud storage which I use for backups already.
DO, as you rightly mentioned, is a lot closer to a straight forward Linux or whatever instance (easier to use). And the prices are very attractive.
I use both and split based on the above. DO is great for hosting web apps / dynamic web sites at a reasonable price. AWS is great for orchestrated clusters / large scale production, or for very low cost static hosting via S3.