For example, in developed countries that may be the case but a birth rate of 2.1 in areas of lower survival e.g. Africa, its less likely due to less healthcare availability.
Yep higher mortality rates - less access to heath care in general and contraceptives - less or later urbanisation so vary need of child labour - less educational female population - lots of reasons that have nothing to do with the 18.8 % drop in UK fertility rates in last 12 years.
I think austerity cuts - less good maternity care - and rising prices are problem with UK.
What we'll do to deal with aging population is keep raising retirement age till it hits 70 or beyond - cut pensions in coming decades - keep high immigration rates and if we have any sense look at building more houses and childcare costs - most of which will be unpopularly with all or large sections of the population.
Some countries have tried pronatalist policies and do get slight bumps in fertility rates but do not get back up to replacement level but think Labour recently ruled these out. Lifting the 2 child benefit cap would alleviate childhood poverty and there is some evidence it's increased abortions.
https://www.bpas.org/media/5hejoy24/forced-into-a-corner-the-two-child-limit-and-pregnancy-decision-making-during-the-pandemic.pdf
Between 2016-2019, the number of abortions performed in England and Wales increased by 11.7%, from 185,596 to 208,384. Over this same period, the number of abortions performed to mothers with 2 or more existing children increased 16.4%, while for women with no existing children or one existing child, the numbers increased by 10.3% and 7% respectively.2 This disproportionate increase in the numbers of abortions performed to women with 2 or more children could be linked to the introduction of the policy.
So I'd say first thing would be getting UK governments to see any fertility decreases as an actual problem.