But what I observed is, the more halving occurs, the ROI or percentage that Bitcoin pumps become smaller. But that is not important to me now. How Bitcoin works and how convenient it to use it, is already good, the percentage gains or ROI is already a bonus.
The Bitcoin having definitely have a positive impact on prices not immediately but after some time as simply it reduce the supply with growing demand which certainly boost the prices.Most trader says that you should invest before halvings to gain profit afterwards and can be seen from this chart :
After halving event there is price surge of 30-40% and at the year end it boost strongly like it went to $20k from $2k in 2017 end.But this pandemic 2020 halving and pandemic boosted btc growth to reach its ATH of $64k but now trading at $57k after correction period.
Your analysis of decrease in percentage of price rise after halving is reducing is true but there are certain reasons for that according to me:
1) Bitcoin has already gained 500% rise this year and reached $60k price levels and it has become hard for retail investors to invest huge funds into it.
2) Bitcoin Total market Cap has already crossed $1.5 trillion and it will need really billion dollars investment to move the market heavily.
3)With huge funds also you will get small share in total supply of 21 million and with increase in demand and prices the share of your Bitcoin portfolio will keep on reducing.
So all these have direct effects on Bitcoin prices but still if you are holding bitcoins for long term then you don't need to worry about these issues as you can easily see 6 figure price chart before 2024 halving which will reduce the rewards to 3.125 and reducing circulating supply and prices even higher with increased demands.