BOOST converter layout check
The basics for all DC/DC converters are:


Minimize the surface area of high dv/dt nodes (like NetD1_2).
Minimize inductance or loop area of high di/dt nodes (U2, D1 and C3).


It seems like you have placed them correctly in a small loop.

However I would  consider adding ceramic capacitors on the output (at/near C3). I don't know what switching frequency you're running the 555 timer at, but fast di/dt pulses are likely not caught well by aluminium capacitors. Ceramic capacitors have a much lower impedance at high frequencies (e.g. >100kHz).

I also wonder if you have connected R4-R5-R6 network correctly on your board.

I would consider getting rid of all thermal reliefs on your board. You may want to keep them if you're not confident in soldering with your equipment, but the relief traces are very small which may play up at high currents.