I don't think so, just a bit of financial remodelling required.
Assumptions
1. use full ISA allowance each year 2 x £20,000
2. dividends at 3.5%
3. ISA growth at 5% pa
Within a few years the cost of the increase in mortgage interest payments will be covered by ISA dividend payments.
Long answer here , short answer below.
Ive just reworked your numbers based on an interest rate for a BTL mortgage at 5.39% - the cheapest rate I could find in a two minute google & applied tax/ tax relief at todays rates & conclude that you would be absolutely bonkers to borrow to invest in an ISA
After 7 years you are still loosing £4k a year & the growth of the ISA has only just crept its value above the mortgage.
In your figures you've paid £280k into the ISA where did the extra £54k magically appear from?
2022/2023 Loan/savings value Interest/dividends
Mortgage 1 £85,000.00 £3,665.20 5.39% less 20% tax relief
Mortgage 2 £69,000.00 £2,975.28 5.39% less 20% tax relief
Mortgage 3 £72,000.00 £3,104.64 5.39% less 20% tax relief
Isa £40,000.00 £945.00 Divi at 3.5% less 32.5tax
Finance cost £(8,800.12)
2023/2024 Loan/savings value Interest/dividends
Mortgage 1 £85,000.00 £3,665.20
Mortgage 2 £69,000.00 £2,975.28
Mortgage 3 £72,000.00 £3,104.64
Isa £82,000.00 £1,937.25
Finance cost £(7,807.87)
2024/2025 Loan/savings value Interest/dividends
Mortgage 1 £85,000.00 £3,665.20
Mortgage 2 £69,000.00 £2,975.28
Mortgage 3 £72,000.00 £3,104.64
Isa £126,100.00 £2,979.11
Finance cost £(6,766.01)
2025/2026 Loan/savings value Interest/dividends
Mortgage 1 £85,000.00 £3,665.20
Mortgage 2 £69,000.00 £2,975.28
Mortgage 3 £72,000.00 £3,104.64
Isa £172,405.00 £4,073.07
Finance cost £(5,672.05)
2026/2027 Loan/savings value Interest/dividends
Mortgage 1 £85,000.00 £3,665.20
Mortgage 2 £69,000.00 £2,975.28
Mortgage 3 £72,000.00 £3,104.64
Isa £217,979.25 £5,149.76
Finance cost £(4,595.36)
2027/2028 Loan/savings value Interest/dividends
Mortgage 1 £85,000.00 £3,665.20
Mortgage 2 £69,000.00 £2,975.28
Mortgage 3 £72,000.00 £3,104.64
Isa £228,878.21 £5,407.25
Finance cost £(4,337.87)
2028/2029 Loan/savings value Interest/dividends
Mortgage 1 £85,000.00 £3,665.20
Mortgage 2 £69,000.00 £2,975.28
Mortgage 3 £72,000.00 £3,104.64
Isa £240,322.12 £5,677.61
Finance cost £(4,067.51)
so take out mortgage on day one for £226,000
year one pay-in ISA £40,000.00
year two pay-in ISA £40,000.00
year Three pay-in ISA £40,000.00
year Four pay-in ISA £40,000.00
year Five pay-in ISA £36,954.00 That’s all that’s left of the cash I borrowed
After 7 years Ive got an ISA worth £240,322 and a £226,000 mortgage
Losing cash at the rate of £4k per year