Can you expand what you mean when you say the security of phone numbers is leagues below email? If someone can gain access to someone's phone, it seems like they would gain access to their email as well.
Cloning your IMEI has nothing to do with the data that is on your phone, so if someone clones your IMEI it does not mean that they have access to any of the apps or data that is on your phone.
SIMs and SIM burners can be purchased trivially on the open market, and cloned without too much difficulty. Although, a social engineering attack on the employee at the cellphone store is a superior method since it automatically gives you a "known good" SIM with the operator's keys, etc.
While there are weaknesses, every mobile phone standard since GSM (not sure about the equivalent for the CDMA world) uses cryptographic authentication, many of which have been subsequently broken, but it's just not true that simple knowledge of a bearer token, transmitted over the air interface, grants you sufficient access to receive somebody else's SMS.
Most practical attacks actually focus on either attacking the core network via SS7 (and making it deliver SMS to the attacker instead of the actual recipient) or on breaking the air interface encryption, which requires you to be physically close to the legitimate recipient while they receive the SMS over the air.
You can change your IMEI to mine right now, and absolutely nothing would happen (other than maybe our phone operator getting mildly confused, if we share one and they're tracking IMEIs for whatever reason).
From that moment onwards, all the 2nd factor SMS OTP go to the attacker.
There are APIs that are provided by mobile operators via aggregators such as Telesign, Prove, Vonage, Twilio etc. that can be used to check if a SIM Swap has happened recently on that phone number. That API is used by fintech companies and others e.g. when they want to check if a fund transfer is to be allowed or flagged up.
SMS are as secure as a letter compared to a postcard.