In transitioning from Tymlos or Forteo to HRT or Bisphosphonates you are basically going from builders to keepers in terms of bone health. If you transition to BHRT and strontium citrate you are going to a keeper and a reinforcer with strontium use.
Research is limited on HRT use instead of Tymlos or Forteo:
The best direct research seems to be using HRT during Forteo treatment. Found with a google search:
"Some studies suggest that starting HRT during anabolic treatment (combination therapy) rather than just after can result in significantly greater BMD increases (e.g., 14.1% in the spine) compared to HRT alone. "
https://pubmed.ncbi.nlm.nih.gov/16418784/
Several research articles and Menopause Societies list HRT as an alternative to bisphosphonates and even to other osteoporosis drugs.
https://theros.org.uk/information-and-support/osteoporosis/treatment/hormone-replacement-therapy/
https://pmc.ncbi.nlm.nih.gov/articles/PMC10009319/
Here is one Mayo discussion of use of HRT after Forteo:
https://connect.mayoclinic.org/discussion/forteo-teriparatide-followed-by-hrt-my-experience/
One of the many advantages of HRT and one not always discussed is:
No Long-Term Retention: Unlike bisphosphonates, which can stay in the bone for over a decade, HRT has a shorter half-life, which may be preferred by patients concerned about long-term drug accumulation.