For me, the only way to do it properly and have a good deliverability is to have a lot of domains, a lot of mailboxes (dont try to have a lot of free gmail, doesnt work on the long shot and you cannot create a lot of gmail from the same IP yourself, they will block the accounts)
You can have cheap domains with Namecheap and then you can have one mailbox from each for free for 2 months (then only 14$/years). Then when you have created thoses mailboxes, it is better to warm them a bit (Mailreach is a bit expensive to warm a lot, but it is a sure service, but maybe you can find cheaper). Then you use a cold mailing software with multi sender feature. You connect them all and you blast a big campaign (Emelia.io with the code UNLIMITED you can connect a lot of mailboxes for 99$/month).
If you want to improve more the deliverability you can also use Spintext in your email.
Anyway, maybe you are looking for a simpler way, maybe there is, but you will have 3-5% open rates, maybe 10% if you are lucky. But with this method, I just did it with 70 mailboxes, sending 7000 a day, I have 72% opens rates.