It's not difficult - most distros have email servers you just install. Most of those use that same email server internally for their mailing lists. It's not much harder (or easier) than setting up any other server.
GMail does not classify an "own email server" as source of spam. Quite apart from anything else how would it know it's an "own email server" as opposed to some companies email server or even a distro email server. They all user the same code.
To get your email server classified as a spammer you have to send spam (mostly). That won't happen if it is just forwarding emails you create, but if it is the work horse behind an email list server or something most of what the list will see is spam and not relaying that to GMail or something becomes very hard indeed.
The reason for the (mostly) above is occasionally somebody will have finger trouble and mark some good email coming for your server as spam. As it happens GMail seems fairly immune to this problem. I guess they must need quite a few people to do make it before it kicks in. Microsoft's servers seem to make the mistake occasionally, and some middle wear boxes can be bloody horrible about it. Fortunately they all seem to provide web pages to fix their mistake you just have to find it.
In a universe where the SORBS DUHL and its ilk never existed, this may be true. In this universe, however, they did and do exist, and running things on one's own machines renders one a third-class citizen.
I'm not sure what evidence you have for this, but I'll provide my own: I run over 30 mail servers on normal ISP links. I can categorically state that's not a problem.
FWIW, I agree was a problem years ago. I suspect it isn't now because ISP's can not longer say what an IP Address is used for, or maybe it's because the DU in that RBL's name stands for Dial Up and this is 2020, not 1999. There is one caveat: you do have to get the DNS PTR records set up.
GMail does not classify an "own email server" as source of spam. Quite apart from anything else how would it know it's an "own email server" as opposed to some companies email server or even a distro email server. They all user the same code.
To get your email server classified as a spammer you have to send spam (mostly). That won't happen if it is just forwarding emails you create, but if it is the work horse behind an email list server or something most of what the list will see is spam and not relaying that to GMail or something becomes very hard indeed.
The reason for the (mostly) above is occasionally somebody will have finger trouble and mark some good email coming for your server as spam. As it happens GMail seems fairly immune to this problem. I guess they must need quite a few people to do make it before it kicks in. Microsoft's servers seem to make the mistake occasionally, and some middle wear boxes can be bloody horrible about it. Fortunately they all seem to provide web pages to fix their mistake you just have to find it.