I think that 4-buss is the best you'll do on a budget.
If you really want 8-buss capability, then IMO an ideally suitable mixer would be
the Fostex 812, which has 8-busses & 12 input channels, as the model number would imply. I suppose it depends exactly the limits of your budget, but the Fostex Model 812 should be one of those relatively affordable 8-buss mixers. I think Fostex made a 20-input/8-buss mixer, as the Model 820.
Then, if you really want HIGH QUALITY, then the Tascam M512 or
M520 mixer would be ideal. It's MUCH higher in features & standard value than ANY Fostex mixer you'll EVER find, and for a bit of a kick up in dollars, is still relatively affordable & WORTH IT.
The Tascam M2600 is a more modern 8-buss mixer, but will be higer priced than the M512. You said you were on a budget.
BACK TO 4-BUSS,... It really depends on the scope of your recording project, as you mentioned 8-simul recording, but IMO a 4-buss mixer could be employed, such as
the Tascam M30 and Tascam M312/
M320. To accommodate 8-simul recording on a 4-buss board, you could use the 4-buss-assign/subgroups to drive 4 tape inputs, and the DIRECT OUTs of the mixer to drive the other 4 tape inputs directly off the other channel strips. Either that, or just drive each of 8 tape inputs with the DIRECT OUTs of each of 8 mixer channels, and leave it at that, with no buss-subgroups used at all. Whether or not I did an adequate job of explaining this, IMO an 8-input/4-buss board such as the M30 would work, with the M312B &
M320B giving you all the more inputs to work with. [The B-models have built-in phantom pwr].
Also, you said you were on a budget but wanted a simple, high quality board, & to me that adds up to only one thing: The Tascam M30, which can be found for avg $125, or even less, depending on the day.
THAT's where the real value is.
However, being that you're running the R8, you may want to stick with an all-Fostex system, which would then bring me back to the Fostex Model 812 & 820. That would probably be the most suitable solution for you,... an all-Fostex solution.
I just happen to think Tascam mixers are of a higher quality & better features than Fostex mixers, overall, but the Fostex mixer solution is there if you want it, and it's an adequate solution that's well matched to your existing R8.