Hi,
I waited for someone else to add an answer for you as i don't use LockBox, and have no experience with it, sources i looked at
https://github.com/TurboPack/LockBox
https://github.com/TurboPack/LockBox3
Now to what i see (without building or compiling anything):
1) Extracted the private key from your post and pasted it on ASN1 decoder and here the result
https://lapo.it/asn1js/#MIICXAIBAAKBgQDFoP5AJIv1KFGRpv_Uw7drFXjWbZG6wNsO7P58ocZIcxyKGU6uTgXw8N1IvTmd9yXRSdcb2fCWB7J_QUQDJQ3YuuXSOQCVOdi8Wy9UoZ5jNdqtZ6CMCvnK_v4Wy38ZhrB0CRkeiuyjmUdfQhe8mh3pE3iFBusYd1TVCxQt3VBkqQIDAQABAoGAaYBaeo-ID6YodWL7a-_XeNkLmxz_EP1nc_5clNgf7AlXkPmVoUORtGBBIVWy7ntDuwh6Ryn_X3hYd8q1riAX1UwVuUduOENmgyzmO1rRIoB_17vzYwVMYOB2h-qbxEqjg4dUfk_1occyDwpehWel-1NIgvQLNYLcn2JXxkAyrMkCQQD37-3Y8sjYxwApgiIClsCjrla73cS_QwzArGEnOjBs86LyzCc0pNzmP2OD0a9VlD3k6dMnhT2Oj-2knZs8dUlHAkEAzA4_mQeFvdiKIkzUBECn3w9Ylu2IfpKnQt_0EFUENxS9ONZ1jj4pzDBfZosgwnE1GiECELM3R_6Pzl-uIGrajwJBALm5HG3az-CykMiHFnrh-kOiII5xvSOYUkEx30THLecvSeyeSPACXwaKjTz9IV31wbdsACQmhsn3vogFF3feU5kCQARP9MYeI5RshBbPeteQKjwLjfq6kFzkaoZ-RyElOs6TMKCH37oe1DFNgGahYBLb45xmwC1sLCnoVk-tM_fZaj8CQGQyIlxwbgNBBdV3wnmtX9yPDflOsjpo3FuBMOu3nZADKEpmTXFgdwP4oMMbCmDvH3dav92LE5JN1cPik9z0Piw
And that is correct and valid RSA key, yet clicked and browsed the sources on github, there is no ASN1 decoder in the sources, the closest thing is using OpenSSL to do the loading of the private key, or to be more accurate to decode the private key to usable format by TP.
2) searching for KJUR.crypto.Cipher.decrypt, found this
https://kjur.github.io/jsrsasign/api/symbols/KJUR.crypto.Cipher.html
So the encryption indeed is RSAOAEP, (RSA encryption is completely different from RSAOAEP for future references don't mix them), also check the sources and its look like TP can do RSAOAEP
https://github.com/TurboPack/LockBox3/blob/master/run/RSA/uTPLb_RSA_Primitives.pas#L116
And that what you should use, so your code is using "Signatory: TSignatory" in complete wrong path
3) Also important the JS code does the decryption, and as usual decryption parameters way less than encryption, to perform RSAOAEP encryption and decryption sometimes you need the default parameters and sometimes you need to figure if there is some default must be set before the decryption.
3) Couldn't find useful examples for you but, so others may help here, or you can start by looking at
https://github.com/TurboPack/LockBox3/blob/master/test/uLockBox_RSA_TestCases.pas#L466
Notice there codec and no TSignatory, but the most important (for me at least) these keys are not decodable as they are not ASN1, from the sources they look like custom format specific to LockBox
Here SO question, close enough to your problem
https://stackoverflow.com/questions/68186850/lockbox-3-encrypt-rsa-with-public-certyficate
Again couldn't find what can solve your problem in full
Suggestion
With above i hope you have better understanding what is your problem actually is, it is loading the private key as first step, then perform the right decryption with RSAOAEP, and that is it, so either try with OpenSSL to load and decrypt or look for different library.
And good luck !