Those were the days
I left out my 8-bit times, but this was a little after that Altair system.
I played with a couple of 6802 and 6502 evaluation boards, the kind of A4-sized PCB with a hex keyboard and 7-segment display and a monitor program in ROM, e.g. during internship at the TELEX exchange at our telecom company and at Philips, but my first own 8-bitter was a 4KB TRS-80 model 1 that I extensively upgraded and modified with homebrew boards using the same techniques
It was built up with 64K RAM, a SD/DD 5"/8" floppy controller (I had 3 surplus 8" floppy drives and two 5" drives I bought new), graphics, and in the end even a 5MB harddisk with SASI controller. It could run CP/M 80 2.2 besides the standard TRS-80 OS.
Like you I spent thousands of guilders (~DM) on that
I programmed it mainly in Z80 assembler (as that was the only way to get some performance out of it), e.g. wrote a monitor program, the BIOS and even a command processor extension for CP/M that used overlays just like the original TRS-80 OS.
With the Atari ST the fun was also in the homebrewing, I also built a harddisk interface for that (before Atari had it available in the shops) and then the Z8530 SCC interface for packet radio that I also used with the PACSATs and UoSATs (with G3RUH 9k6 and PSK 1k2 modem).
(I wrote my own software for the PACSAT broadcast and file transfer protocols and automated my entire station to do all the tracking, uploading and downloading without me having to touch anything, much to the dismay of some users of the "official" software who always found me connected even at 02:00 before they could press their connect button...)
This software was used for some time by the BBS systems that forwarded their bulletins via the satellite.
But I did also an implementation of NET/ROM (after TheNET of course) on the Atari ST as part of KA9Q NET. All that software I could cross-compile for MS-DOS so others with a PC clone could use it as well, and a local amateur PA0HZP developed a Z8530 card for ISA bus and sold a lot of kits. Some others made similar cards. Most packet nodes and BBSes here used this setup.
Those were fun times, but for me the computing really became interesting when I got my Linux system running and suddenly it was not a 1-program-at-a-time thing anymore. Before that, I had two Atari ST systems at home, one for running the amateur radio system (with a TRS-80 mini color computer, later an EPSON PX-8 alongside to run the W3IWI tracking program) and another one to do development. With Linux this was no longer required and everything, packet, tracking, BBS, and development, by then also internet browsing/mail etc, ran on one single system. But not so much homebrewing anymore, just standard PC hardware with some oddities attached via serial ports. E.g. the rotator via an AMSAT-DL tracking interface controlled via RTS and DTR of a serial port
The other systems more and more became relics that I never discarded because they had cost so much money, but are no longer in use. I still have most of it in storage boxes...