The technical term for the kind of sub-CA you're asking for is Technically Constrained TLS Subordinate CA Certificate.
It is true that the Baseline Requirements allow institutions like Let's Encrypt to issue such certificates, and it's even true that the audit requirements for such sub-CAs are less than the audit requirements for "normal" intermediates (like our E5 and R10).
However, those audit requirements do still exist. The BRs, in Section 8.1, say that
Certificates that are capable of being used to issue new certificates MUST either be Technically Constrained... as well as audited in line with Section 8.7 only, or...
So let's take a look at those requirements from Section 8.7:
During the period in which a Technically Constrained Subordinate CA issues Certificates, the CA which signed the Subordinate CA SHALL monitor adherence to the CA's Certificate Policy and the Subordinate CA's Certification Practice Statement. On at least a quarterly basis, against a randomly selected sample of the greater of one certificate or at least three percent of the Certificates issued by the Subordinate CA, during the period commencing immediately after the previous audit sample was taken, the CA shall ensure all applicable CP are met.
So if we give you a sub-CA, then every quarter we need to audit at least 3% of all certificates you've issued, and ensure that you're complying with our CP/CPS. What does that mean in practice?
(nota bene: everything from here on out is hypothetical)
For starters, you almost certainly can't comply with our CP/CPS as written: it is a very specific document that would take a significant amount of work for you to comply with. Cherry-picking one example, you'd have to ensure that your key material is generated by and stored on a FIPS-qualified HSM.
We'd also have to employ additional personnel to perform those quarterly audits of your certificates and of your compliance with our CP/CPS. And since we'd be putting our own reputation as a CA on the line, we'd need a strong contract laying out what happens if you deceive or evade the audit process. Both auditors and lawyers cost money, so we'd probably need you to pay us for the privilege of having a sub-CA.
And then we need to do all of that for every subscriber who requests this profile.
At that point, we've drastically departed from the mission of Let's Encrypt: to make TLS free and ubiquitous by automating it. We can't automate audits, we can't automate contracts, and we certainly can't do those things for free. So this is a service that would be impossible for ISRG to provide.