On your question whether treasury bills and bonds are a good investment then it all depends on how much money you have to invest.
If your investment volume is low, then you would rather invest in a grocery shop with returns of about 30% than go with treasury bonds and bill with returns of 10%.
With Ksh. 100,000 investment in T bills per year with interest rate of 10%, your interest will be about Ksh. 7,700 after tax.
If your investment volume is high and you don’t have time or energy to be monitoring your business then treasury bonds or bills are good choice for you.
If are ailing or are at your 70s, you probably have no energy of going to CBD every day to monitor your business and therefore treasury bonds/bill are your best choice as they give you peace of mind.
You can use the Treasury Bills Pricing Calculator and Treasury Bonds Pricing Calculator to calculate your average returns per investment
T Bill calculator: https://www.centralbank.go.ke/bills-bonds/treasury-bills/
NOTE: Interest rates vary depending on tenor and day.