Good question…just be sure to check who’s the supplements affect the kidneys…strontium could stress them.

no strontium is not a substitute for calcium.
I am a strontium user; I spend all day long making sure that I eat and consume enough calcium and then take my strontium the last thing at night. In fact, the main priority I have with my strontium use and the one which my endocrinologist left me with is to make sure that I take enough calcium daily before my strontium.

Think of it this way, strontium is more of a scaffolding for the osteoporosis holes in the bone, especially found in new bone, but then calcium is needed to adhere before bone grows.
Strontium does not replace calcium; it works with calcium to grow bone.

Plus, strontium research does not say that strontium stresses kidneys. Research does say that most drugs and supplements including strontium should be used carefully when kidneys are damaged. This is because the kidneys can not do their job well enough sometimes to clean the body of supplements and drugs and indeed many nutrients found in food. So kidney concerns when you have diseased kidneys extend to all osteoporosis drugs, most supplements and many of the nutrients in our diet.

Research:
Long-term Phase 3 trials (SOTI and TROPOS) of strontium ranelate—which provides 680 mg of elemental strontium—monitored thousands of patients for up to 5 years. These studies found no clinically relevant changes in renal function compared to placebo groups.
Animal Toxicity Studies: In rats and chickens, even extremely high doses of strontium (up to 166 mg/kg/day, which would be over 11,000 mg for a human) showed no histological changes or weight changes in the kidneys, provided calcium and vitamin D levels were adequate.
Acute Kidney Injury (AKI) Correlation: Some specialized research has linked high serum (blood) strontium levels to an increased risk of acute kidney injury specifically after cardiac surgery. This is typically interpreted as a marker of the body's inability to clear the mineral rather than the mineral causing the damage.

Said another way:
Most clinical evidence suggests the relationship is a correlation of function, not necessarily a causative one where strontium harms or heals the kidneys.
So the problem arises with the kidneys are they healthy, not does strontium cause damage to the kidneys.

@pat9892 Strontium (an element chemically similar to calcium, denser) is not a substitute for calcium. There doesn't seem to be a lot of research on strontium, but here is an NIH publication from 2021. Written to doctors, but does contain the processes of Ca and Sr on bone growth, remodeling, and loss. https://pmc.ncbi.nlm.nih.gov/articles/PMC8235140/